High-efficiency encoder and video information recording/reproducing apparatus

ABSTRACT

In a high-efficiency encoder which performs motion-compensation prediction, an intra-field is set every n fields. The presence of a scene change is detected. When a scene change occurs, a reference picture of motion-compensation prediction is switched, or the field immediately after the scene change is set as an intra-field.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a digital signal recording/reproducingapparatus such as a video tape recorder (hereinafter, abbreviated as“VTR”), a video disk player and an audio tape recorder in which videoand audio signals are recorded and reproduced in the digital form, andparticularly to an apparatus which performs motion-compensationprediction on a video signal for compression-encoding.

[0003] 2. Description of the Related Art

[0004] In a digital VTR for home use, data compression is indispensablein view of cost and hardware size. Hereinafter, therefore, datacompression will be described taking mainly a digital VTR for home useas an example.

[0005]FIG. 1 is a schematic block diagram showing the structure of adigital VTR for home use. The reference numeral 900 designates an inputterminal through which an analog video signal such as a televisionsignal is input. The reference numeral 901 designates an A/D converterwhich converts the analog video signal into a digital video signal, 902designates a data compressor which compresses the digital video signalto reduce the information amount of the signal, 903 designates anerror-correction encoder which adds error-correcting codes to the codedsignal so that errors are corrected in the reproduction, 904 designatesa recording modulator which, in order to perform the recording,modulates the signal to codes suitable for the recording, 905 designatesa recording amplifier which amplifies the record signal, and 906designates a magnetic tape on which the record signal is recorded to bestored. The reference numeral 907 designates a head amplifier whichamplifies a signal reproduced from magnetic tape 906, 908 designates areproduction demodulator which demodulates the reproduced signal, 909designates an error-correction decoder which performs error-correctionon the reproduced and demodulated signal using the error-correctingcodes, 910 designates a data expander which reconstructs the compresseddata to its original form, 911 a D/A converter which converts thedigital video signal into an analog video signal, and 912 designates anoutput terminal.

[0006] Next, the data compressor (high-efficiency encoder) 902 will bedescribed. FIG. 2 is a block diagram of the high-efficiency encoderwhich employs one-way motion-compensation inter-frame prediction. Thereference numeral 1 designates an input terminal for a digital videosignal, 2 designates a blocking circuit which segments the input digitalvideo signal, 3 designates a subtracter which outputs as a differenceblock a difference signal between an input block and a prediction block,4 designates a difference power calculator which calculates the power ofthe difference block, 5 designates an original power calculator whichcalculates the AC power of the input block, 6 designates a determinerwhich compares the difference power with the original AC power todetermine whether the current mode is a prediction mode or an intramode, 7 designates a first switch which selectively outputs an encodedblock in accordance with the determined mode, 8 designates a DCT circuitwhich performs on the encoded block the discrete cosine transform(hereinafter, abbreviated as “DCT”) that is an orthogonal transform, 9designates a quantizing circuit which quantizes a DCT coefficient, 10designates a first encoder which performs the coding suitable for atransmission path, and 11 designates the transmission path.

[0007] Reference numeral 12 designates an inverse quantizing circuitwhich performs inverse-quantization on the quantized DCT coefficient, 13designates an inverse DCT circuit which performs the inverse DCT on theinverse-quantized DCT coefficient, 14 designates an adder which adds aprediction block to the decoded block that is an output signal of theinverse DCT circuit 13 to generate an output block, 15 designates avideo memory which stores output blocks in order to performmotion-compensation prediction, 16 designates an MC circuit whichperforms motion estimation from a motion-compensation search blocksegmented from a past image stored in the video memory 15 and thecurrent input block, and performs motion-compensation prediction, 17designates a MIX circuit which combines a motion vector with a modesignal determined by the determiner 6, 18 designates a second encoderwhich codes the output of the MIX circuit 17, and 19 designates a secondswitch which switches the prediction blocks in accordance with the modedetermined by the determiner 6. The difference power calculator 4,original power calculator 5, determiner 6, inverse quantizing circuit12, inverse DCT circuit 13, adder 14, video memory 15, MC circuit 16 andsecond switch 19 constitute a local decoding loop 20.

[0008] Then, the operation will be described. Irrespective of anintra-field in which motion-compensation prediction is not performed ora prediction-field (inter-field) in which motion-compensation predictionis performed, input digital video signals are divided by the blockingcircuit 2 into input blocks in the unit of m [pixels]×n [lines] (where mand n are positive integers), and segmented. In order to obtain adifference block, the subtracter 3 calculates the difference in the unitof pixel between an input block and a prediction block. Then, the inputblock and the difference block are input into first switch 7. Thedifference power calculator 4 calculates the difference power of thedifference block. On the other hand, the original power calculator 5calculates the original AC power of the input block. The two calculatedpowers are compared with each other by the determiner 6 to control thefirst switch 7 so that the block having the smaller power is selected asthe encoding subject. More specifically, when the difference power issmaller than the original AC power, the determiner 6 outputs aprediction mode signal, and in contrast with this, when the original ACpower is smaller than the difference power, the determiner 6 outputs anintra mode signal.

[0009] The first switch 7 outputs the input block or the differenceblock as an encoded block in accordance with the mode signal determinedby the determiner 6. When the processed image is in the intra-field,however, the first switch 7 operates so that all of the encoded blocksare output as input blocks. FIG. 3 illustrates this switching operation.The ordinary mode is a mode where, in a step of motion-compensationprediction which is completed in four fields as shown in FIG. 4, firstfield F1 of the four fields is always an intra-field and the succeedingsecond, third and fourth fields F2, F3 and F4 are prediction-fields.

[0010] The encoded block selected by the first switch 7 is convertedinto DCT coefficients by the DCT circuit 8, and then subjected to theweighting and threshold processes in the quantizing circuit 9 to bequantized to predetermined bit numbers respectively corresponding to thecoefficients. The quantized DCT coefficients are converted by the firstencoder 10 into codes suitable for the transmission path 11 and thenoutput to the transmission path 11.

[0011] The quantized DCT coefficients also enter into the local decodingloop 20, and the image reproduction for next motion-compensationprediction is performed. The quantized DCT coefficients which haveentered into the local decoding loop 20 are subjected to the inverseweighting and inverse quantizing processes in the inverse quantizingcircuit 12. Then, the DCT coefficients are converted into a decodedblock by inverse DCT circuit 13. The adder 14 adds the decoded block toa prediction block in the unit of pixel to reconstruct the image. Thisprediction block is the same as that used in the subtracter 3. Theoutput of the adder 14 is written as an output block in a predeterminedaddress of the video memory 15. The memory capacity of the video memory15 depends on the type of the employed predictive method. Assuming thatthe video memory 15 consists of a plurality of field memories, thereconstructed output block is written in a predetermined address. Ablock which is segmented from an image reconstructed from past outputblocks and is in the motion estimation search range is output from thevideo memory 15 to the MC circuit 16. The size of the block in themotion estimation search range is i [pixels]×j [lines] (where i≧m, j≧n,and i and j are positive integers). Data in the search range from thevideo memory 15 and an input block from the blocking circuit 2 are inputto the MC circuit 16 as data, thereby extracting motion vectors. As amethod of extracting motion vectors, there are various methods such asthe total search block matching method, and the tree search blockmatching method. These methods are well known, and therefore theirdescription is omitted.

[0012] The motion vectors extracted by the MC circuit 16 are input tothe MIX circuit 17, and combined therein with the mode signal determinedby the determiner 6. The combined signals are converted by the secondencoder 18 into codes suitable for the transmission path 11, and thenoutput together with the corresponding encoded block to the transmissionpath 11. The MC circuit 16 outputs as a prediction block signals whichare segmented from the search range in the size (m [pixels]×n [lines]),which is equal to that of the input block. The prediction block to beoutput from the MC circuit 16 is produced from past video information.The prediction block is supplied to second switch 19, and output fromthe respective output terminal of the switch in accordance with thefield of the currently processed image and the mode signal of thedecoded block. Namely, the prediction block is output from one of theoutput terminals of the second switch 19 to the subtracter 3 inaccordance with the processed field, and from the other output terminalin accordance with the mode signal of the current decoded block and theprocessed field.

[0013] As a predictive method used in such a circuit block, for example,the method shown in FIG. 4 may be employed. In this method, anintra-field is inserted after every three fields, and the threeintermediate fields are set as prediction-fields. In FIG. 4, first fieldF1 is an intra-field, and the second, third and fourth fields F2, F3 andF4 are prediction-fields. In the prediction by this method, second fieldF2 is predicted from first field F1 which is an intra-field, third fieldF3 is predicted in a similar manner from first field F1, and fourthfield F4 is predicted from reconstructed second field F2.

[0014] Initially, first field F1 is blocked in the field and subjectedto the DCT. Then, first field F1 is subjected to the weighting andthreshold processes and quantized, and thereafter encoded. In the localdecoding loop 20, the quantized signals of first field F1 are decoded orreconstructed. The reconstructed image is used in motion-compensationprediction for second and third fields F2 and F3. Then,motion-compensation prediction is performed on second field F2 usingfirst field F1. After the obtained difference block is subjected to theDCT, encoding is performed in a similar manner as in first field F1. Inthis case, when the AC power of the input block is smaller than thepower of the difference block, the input block in place of thedifference block is subjected to the DCT, and thereafter encoding isperformed in a similar manner as in first field F1. Second field F2 isdecoded and reconstructed in the local decoding loop 20 in accordancewith the mode signal of each block, and then used in motion-compensationprediction for fourth field F4. In a similar manner as in second fieldF2, using first field F1, motion-compensation prediction and encodingare performed on third field F3. Motion-compensation prediction isperformed on fourth field F4 using second field F2 reconstructed in thevideo memory 15, and then, fourth field F4 is encoded in a similarmanner as in third field F3. Also in third and fourth fields F3 and F4,when the AC power of the input block is smaller than the power of thedifference block, the input block in place of the difference block issubjected to the DCT, and thereafter encoding is performed in a similarmanner as in first field F1.

[0015] For example, the digital VTR for home use shown in FIG. 1 isexpected to achieve the high image quality and high tone quality. Inorder to realize this, it is essential to improve data compression,i.e., performance of high-efficiency encoder. Therefore, there arisefollowing problems in the above-described conventional predictivemethod.

[0016] In such a predictive method, since motion-compensation predictionis performed using the video data of the one preceding field or frame,there arises a first problem that the capacity of the field memory orframe memory is increased and the hardware is enlarged in size.

[0017] In the conventional predictive method, when a scene change onceoccurs in the unit of frame, it is difficult during encoding of theimage after the scene change to perform the compression according tomotion-compensation prediction from the reference picture which wasobtained before the scene change, thereby causing a second problem thatthe total amount of codes is increased. If inter-framemotion-compensation prediction is performed on the whole sequentially inthe temporal direction, it may be possible to suppress the increase inthe data amount to a minimum level even when a scene change occurs. Inthe case of encoding interlace images without scene change and with lessmotion, however, there is a tendency that the data amount is increasedas a whole. In a predictive method in which third and fourth fields F3and F4 are adaptively switched from first, second and third fields F1,F2 and F3 as shown in FIG. 5, there is a drawback that the capacity ofthe field memory or frame memory is increased and the hardware isenlarged in size. FIG. 6 shows the data amount and S/N ratio of aluminance signal, for example, when an image A with scene changes isprocessed by the predictive method of FIG. 4 or the predictive method ofFIG. 5. In the image A, a scene change occurs in the unit of frame. FIG.6 shows also the data amount and S/N ratio of a luminance signal in whenan image B without scene changes is processed by the predictive methodof FIG. 4 or the predictive method of FIG. 5. In this case, for theimage A with scene changes, the predictive method of FIG. 5 isadvantageous, and, for the image B without scene changes, the predictivemethod of FIG. 4 is advantageous.

[0018] In the case that the encoding is done by performing prediction asin the prior art, there is a third problem that, when a scene changeoccurs in a step of a motion-compensation prediction process, thequality of the image immediately after the scene change is deteriorated.This problem is caused as, owing to the scene change,motion-compensation prediction which largely utilizes time correlationis unsatisfactorily performed, thereby increasing the information amountbeing generated. The information amount generated in this way cancompare with the information amount of a usual intra-field in the level.For the generated information amount, the field having this informationamount is used as the prediction-field, and therefore, the informationamount is compressed to the level of the information amount of theprediction-field, resulting in the image quality of the field after ascene change being substantially deteriorated. FIG. 7 shows a change ofthe information amount of images for five seconds when encoding isperformed by a conventional predictive method. In this case; the averagefor five seconds is less than 20 [Mbps], but a scene change exists at aposition A, thereby increasing the information amount. The change of theS/N ratio in this case is shown in FIG. 8. Although there is no greatdeterioration in the portion of the scene change, the decrease of theinformation amount makes the S/N ratio deteriorated. When that field isused in the next motion-compensation prediction, it is necessary toperform motion-compensation prediction on the image with thedeteriorated image quality and the reduced time correlation, the resultbeing that the information amount being generated is again increased.This vicious cycle continues until the next refresh field is processed.If deterioration of the image quality occurs in this way, even though itis immediately after a scene change, that means a digital videorecording/reproducing apparatus which is required to have a high imagequality fails to make the best use of the performance. In a VTR for homeuse which is one of the digital video recording/reproducing apparatuses,for example, functions such as a trick play and edition areindispensable, and, when such a function is performed, remarkabledeterioration occurs in the image quality.

[0019] As conventional VTRs for home use of helical scanning type, thereare VHS type, β type and 8-mm type VTRs. Hereinafter, a VTR of 8-mm typewill be described as an example of a prior art. FIG. 9 is a diagramshowing the tape format according to the 8-mm VTR standard, and FIG. 10is a diagram showing the format of one track. FIG. 11 is a diagramshowing the relationship between a rotary head drum and a magnetic tapewound around it, and FIG. 12 is a graph showing the frequency allocationof each signal according to the 8-mm VTR standard. In an 8-mm VTR forthe NTSC system or PAL system, a video signal is recorded by a colorundermethod which is a basic recording method for VTRs for home use. Theluminance signal is frequency-modulated with a carrier of 4.2 to 5.4MHz, chroma signal subcarrier is converted into a low frequency signalof 743 kHz, and the both signals are subjected to the frequencymultiplex recording. The recording format on a tape is as shown in FIG.9. All signals required for a VTR at least including a video signal(luminance signal, color signal), audio signals and tracking signals aresubjected to the frequency multiplex recording by the rotary video head.The frequency bands are shown in FIG. 12.

[0020] In FIG. 9, magnetic tracks 401 and 402 of a video signal trackportion 410 are tracks for a video signal, and each corresponds to onefield. Magnetic tracks 403 and 404 indicated with oblique lines in anaudio signal track portion 411 are magnetic tracks for audio signals. Acue track 405 and audio track 406 for a fixed head are respectively seton the both edges of the tape. Since the control track on the tape edgeis not used in an 8-mm VTR, this track can be used as the cue track forperforming specific point searching, addressing the contents ofrecording or the like. The width of one track (track pitch) is 20.5 μm,and is slightly greater than that in the economy recording mode of βtype and VHS type (19.5 μm in β−7, 19.2 μm in the 6-hour mode of VHS).No guard band for preventing a crosstalk from occurring is set betweentracks. Instead, azimuth recording using two heads is employed in orderto suppress a crosstalk.

[0021] Next, a specific example of the operation of a conventionalapparatus will be described with reference to FIGS. 13 to 16. FIG. 13 isa block diagram of a conventional VTR. A video signal given to a videosignal input terminal 201 is supplied to a video signal processingcircuit 203 and also to a synchronizing signal separating circuit 204.The output signal of the video signal processing circuit 203 is fedthrough gate circuits 205 and 206 to adders 213 and 214. In contrast, avertical synchronizing signal which is an output of the synchronizingsignal separating circuit 204 is supplied to delay circuits 207 and 208.The Q output of the delay circuit 207 which combines with thesynchronizing signal separating circuit 204 to constitute head switchpulse generation means is supplied as a gate pulse to the first gatecircuit 205 and also to a fourth gate circuit 212 which will bedescribed later. The {overscore (Q)} output is supplied as a gate pulseto the second gate circuit 206 and also to a third gate circuit 211which will be described later. The output signal of the delay circuit208 is supplied to a time-base compressing circuit 209 and also to anerasing current generator 240.

[0022] An audio signal given to an audio signal input terminal 202 issupplied through the time-base compressing circuit 209, a modulatingcircuit 210 and a switch 241 for switching between the recording and theerasing, to the third and fourth gate circuits 211 and 212. The outputof the erasing current generator 240 is supplied through the switch 241to the third and fourth gate circuits 211 and 212. The output signals ofthe third and fourth gate circuits 211 and 212 are supplied to theadders 213 and 214, respectively. The output signal of the adder 213 isgiven to a rotary transformer 217 through a changeover switch 215 forswitching between the recording and the erasing. The output signal ofthe rotary transformer 217 is given to a rotary magnetic head 221through a rotation shaft 219 and a rotary head bar 220, so that arecording current or an erasing current flows into a magnetic tape 223.

[0023] The output signal of the adder 214 is given to a rotarytransformer 218 through a switch 216 which is used for switching betweenrecording and the erasing and is inter-locked with the switch 215. Theoutput signal of the rotary transformer 218 is given to another rotarymagnetic head 222 through the rotation shaft 219 and the rotary head bar220, so that a recording current or an erasing current flows into themagnetic tape 223. The magnetic tape 223 is guided by guide posts 224and 225 placed on the either sides of a table guide drum 226 which hasrotary magnetic heads 221 and 222 built in, and is run at a constantspeed in the direction of arrow 227, by a well known magnetic taperunning device (not shown) which consists of capstans and pinch rollers.The table guide drum 226 may have a well known structure, and thereforeits specific description is omitted.

[0024] In the reproduction process, a signal reproduced by the rotarymagnetic head 221 is supplied to a separating circuit 228 through therotary head bar 220, the rotation shaft 219, the rotary transformer 217and the switch 216. On the other hand, a signal reproduced by the rotarymagnetic head 222 is supplied to a separating circuit 229 through therotary head bar 220, the rotation shaft 219, the rotary transformer 218and the switch 216. One of the outputs of the separating circuit 228 andone of the outputs of the separating circuit 229 are supplied to anadder 230. The other output of the separating circuit 228 and the otheroutput of the separating circuit 229 are supplied to an adder 231. Theoutput signal of the adder 230 is supplied to a video signal outputterminal 233 through a video signal processing circuit 232. In contrast,the output signal of the adder 231 is supplied to an audio signal outputterminal 237 through time-base correcting circuit 234, a demodulatingcircuit 235 and a time-base expanding circuit 236.

[0025] Then, the operation will be described. A video signal given tothe video signal input terminal 201 is converted into an FM signal bythe video signal processing circuit 203. When the video signal includesa chrominance signal, the chrominance signal is converted into a lowfrequency signal of less than about 1.2 MHz. There will be no problemthat, for example, the phase of the chrominance signal is shifted by 90deg. or inverted every 1H (horizontal scanning interval) as means foreliminating an adjacent color signal. This is a technique of eliminatinga crosstalk between tracks with using the line correlation ofchrominance signal. Such processed video signal is supplied to first andsecond gate circuits 205 and 206.

[0026] On the other hand, since the video signal is given also to thesynchronizing signal separating circuit 204, a vertical synchronizingsignal is obtained at the end of the output of the circuit. The verticalsynchronizing signal is supplied to the delay circuits 207 and 208. Thedelay circuit 207 has functions of dividing an input signal into a halffrequency and delaying a signal. From the ends of Q and {overscore (Q)}outputs of the delay circuit 207, pulse signals Q and {overscore (Q)}for switching the heads and shown in FIGS. 14(b) and 14(c) are suppliedto the first and second gate circuits 205 and 206, respectively. Inorder to clarify the relationship in phase between these pulse signals Qand {overscore (Q)} and the input video signal, the waveform of theinput video signal is shown in FIG. 14(a). From the ends of outputs offirst and second gate circuits 205 and 206, the processed video signalsare output as shown in FIGS. 15(a) and 15(b) during the periods in whichthe pulse signals Q and {overscore (Q)} are at H level. These signalsare added to a modulated compressed audio signal and erasing signalwhich will be described later, by adders 213 and 214, and then suppliedto switches 215 and 216. The compressed audio signal is subjected tomodulation suitable for the tape and head system (preferably, the pulsecode modulation (PCM), or FM, PM, AM or the like, or in certain cases,the non-modulation AC bias recording), by the modulating circuit 210.Particularly, PCM is advantageous because a high S/N ratio can beexpected and well known error correction means can be used for thedrop-out, etc. The modulated compressed audio signal is given throughthe switch 241 to the third and fourth gate circuits 211 and 212 towhich the pulse signals Q and {overscore (Q)} are respectively supplied.These gate circuits 211 and 212 output the compressed audio signal tothe adders 213 and 214 during the periods in which the pulse signals Qand {overscore (Q)} are at H level.

[0027] The erasing current generator 240 generates an erasing current ofa certain frequency (for example, 100 kHz). The timing of starting theoscillation of the erasing current is controlled by a trigger signal Twhich is obtained by delaying the vertical synchronizing signal in thedelay circuit 208. The erasing current is output through the switch 241to the third and fourth gate circuits 211 and 212 to which the pulsesignals Q and {overscore (Q)} are respectively supplied, and supplied tothe adders 213 and 214. In the same manner as the recording ofcompressed audio signals, during the periods in which {overscore (Q)}pulse signals {overscore (Q)} and Q are at H level. FIGS. 16(a) and16(b) shows the waveforms of the output currents of the adders 213 and214, i.e., time-multiplexed signals of a processed video signal A and aprocessed audio signal B or the erasing signal. These signals aresupplied via the above-mentioned paths to the rotary magnetic heads 221and 222, whereby the magnetic pattern of a tape shown in FIG. 9 isobtained.

[0028] During the reproduction process, the moving contacts of theswitches 215 and 216 are positioned at fixed contacts P. This allows thetwo-channel reproduction signal reproduced by the rotary magnetic heads221 and 222 to be respectively transmitted through the rotary head bar220, the rotation shaft 219, the rotary transformers 217 or 218 and theswitches 215 or 216, and to be respectively separated into a videosignal and an audio signal on the time-base in separating circuits 228and 229. The separated video signals are converted by the adder 230 intoa one-channel video signal which is continuous in terms of time, andthen supplied to the video signal processing circuit 232. The videosignal processing circuit 232 reconstructs the original video signalfrom the input signal, and outputs the reconstructed signal to the videosignal output terminal 233. On the other hand, the separated audiosignals are converted into a one-channel of signal by the adder 231, andthen supplied to the time-base correcting circuit 234. The time-basecorrecting circuit 234 consists of a semiconductor memory device such asa CCD (charge-coupled device) and a BBD (bucket brigade device), andeliminates time-base variations (so-called jitter and skew distortion)of the tape and head system. The output signal of the time-basecorrecting circuit 234 is demodulated to the original compressed audiosignal by the demodulating circuit 235. The demodulated signal is thenconverted into the original audio signal by the time-base expandingcircuit 236 consisting of a semiconductor memory device such as a CCDand a BBD, and output to the audio signal output terminal 237.

[0029] As described above, in an 8-mm VTR, video signals and audiosignals for one field are recorded on and reproduced from one track on atape.

[0030]FIG. 17 is a block diagram showing the configuration of aconventional video information recording/reproducing apparatus. In FIG.17, a digital VTR of the D1 or D2 method which is used for business orbroadcasting use is shown. The reference numeral 101 designates an A/Dconverter which converts an analog video signal into a digital videosignal, 102 designates an error-correction encoder which addserror-correcting codes, 103 designates a modulator which modulates thedigital signal to a signals suitable for the recording on a magnetictape, 104 designates a rotary head drum, 105 designates a magnetic tape,106 designates magnetic head for recording and reproduction, 107designates a demodulator which demodulates the reproduced signal, 108designates an error-correction decoder which detects and corrects atransmission error, and 109 designates a D/A converter which convertsthe digital video signal into an analog video signals.

[0031]FIG. 18 shows the tape formats of the two methods. In the bothmethods, a video signal and a 4-channel audio signal are recorded indifferent positions in the same track. In the D1 method, an audio signalis recorded in the center of a track, and, in the D2 method, at the endsof a track. When a video signal and an audio signal are recorded in thesame track, components such as a magnetic head and an amplifying circuitwhich are necessary for recording and reproducing can be used in commonfor a video signal and an audio signal, and furthermore, a parity coderequired for the error correction as described later and a circuit forgenerating the parity code can be used in common.

[0032]FIG. 19 shows the overall specifications of the D1 and D2 methods,FIG. 20 shows the specifications of the tape formats, and FIG. 21 showsthe specifications of the tape running systems. The area recordingdensity with guard bands being taken into account is 21.5 μm²/bit in theD1 method, and 16.6 μm²/bit in the D2 method. In the D1 method, guardbands are set between recording tracks, but, in the D2 method, guardbands do not exist. As a result, the track density of the D2 method ishigher than that of the D1 method by about 15%, which contributes to thelong-time recording by the D2 method. On the other hand, when guardbands do not exist, it is more likely to reproduce a signal of anadjacent track in addition to a signal of the track originally intendedto be reproduced. In order to cope with this crosstalk between tracks inthe reproduction process, the D2 method employs the azimuth recordingsystem. Generally, a recording magnetic head and a reproduction magnetichead are so positioned that their head gaps form the equal angle with amagnetic track. If the two head gaps are arranged so as to form an anglewith each other, the level of a reproduced signal shows an attenuationcharacteristic. The azimuth angle θ in the D2 method is about ±15 deg.as shown in FIG. 20. As a result, even if a signal from an adjacenttrack is mixed in signals, to be reproduced the unnecessary component isattenuated. Accordingly, even if guard bands do not exist, the effect ofthe crosstalk is reduced. Since the loss due to the azimuth angle cannotbe expected for DC components, however, signals to be recorded arerequired to have no DC component. Therefore, the D2 method employs amodulation system which does not include DC components.

[0033] In a digital recording, it is not necessary to record a videosignal during the entire period. In a blanking interval, a video signalhas a constant waveform irrespective of the contents of an image. Sincethis waveform can be synthesized after the reproduction, in both the D1and D2 methods, the recording is performed only during the effectivevideo period. Also a color burst signal appearing in a blanking intervalof an NTSC signal can be synthesized after the reproduction. This isbecause the sampling phase in the D2 method is set to the I and Q axesand the phase of the color burst (lagging behind Q axis by (180+33)deg.) can be determined using a reproduced sampling clock.

[0034]FIG. 22 shows the ranges in which pixels can be actually recordedin the D1 and D2 methods. These effective pixels are divided intoseveral segments. In the D1 method, pixels of 50 scanning linesconstitute one segment, and, in the D2 method, pixels of 85 scanninglines constitute one segment. In other words, pixels of one fieldconstitute five segments in the D1 method, and three segments in the D2method.

[0035] When a video signal in a segment is to be recorded, it is dividedinto four channels in the D1 method, and into two channels in the D2method. As a result, the number of pixels per one channel of one segmentis {(720+360×2)/4}×50=360×50=18,000 in the D1 method, and(768/2)×85=384×85=32,640 in the D2 method. Channels are distributed sothat they are uniformly dispersed on a screen. Accordingly, even whenthe characteristics of a specific channel are deteriorated, code errorscaused by this deterioration are not concentrated on one portion of thescreen so as to be inconspicuous. Therefore, the effect of correction onerrors which have not been corrected is also enormous.

[0036] In both the D1 and D2 methods, two kinds of error-correctioncodes which are respectively called an outer code and an inner code areused together. In an actual process of generating outer and inner codes,an operation of rear-ranging the sequence of the codes is performed.This operation is called shuffling. The shuffling disperses the effectof code errors, improves the correction capability, and reduces thedisplay deterioration caused by errors which have not been corrected.The shuffling process consists of the shuffling for one scanning linewhich is performed before the generation of an outer code, and theshuffling which is performed in one sector after the addition of anouter code and before the generation of an inner code. As describedabove, in a VTR of the D1 or D2 method, video signals and audio signalsfor one field are recorded in a plurality of tracks on a tape.

[0037] In order to record all information of standard television signalsof currently used NTSC and PAL systems, in a VTR for home use, thecarrier frequency of an FM luminance signal is raised and the bandwidthand deviation are increased so as to improve the resolution and C/Nratio. However, a VTR for home use still fails to match with a VTR forbusiness use in S/N ratio, waveform reproducibility, etc. The downsizing of a VTR is highly expected to be achieved, and, there aredemands for further improvement in performance as well as realization ofVTR which is light and compact. Hence, it is difficult to attain thedesired performance by only improving the present techniques. On theother hand, in the field of VTRs for business use and broadcasting use,rapid advance in digitalization of an apparatus has been made to achievemultifunction and high performance in the apparatuses, and most of VTRsfor broadcasting use are replaced with digital VTRs. However, a digitalVTR consumes a large amount of tape, which obstacles to achieveprolongation of the recording time and the down sizing.

[0038] Recently, in view of the redundancy of information contained inan image, studies on compressing recorded information have been activelyconducted, and the application of the results of these studies to a VTRis being examined. It is expected to realize a VTR which is compact andlight, has high image quality and can operate long-time by achievinghigh image quality, and reduction of tape consumption due to highdensity recording and information compression which are inherent in thedigital recording.

[0039]FIG. 23 shows a communication apparatus of a high-efficiencyencoded video information compression method (according to CCITT H. 261,etc.) which is used in the field of communication including a videotelephone and a video conference. The reference numeral 101 designatesan A/D converter which converts an analog video signal into a digitalvideo signal, 110 designates a high-efficiency code encoder whichcompression-encodes a video signal, 112 designates a buffer memory whichis used for delivering generated compressed codes at a constant speed,102 designates an error-correction encoder which adds error-correctingcodes, 103 designates a modulator which modulates the digital signal toa transmission signal suitable for the communication, 114 designates atransmission path, 107 designates a demodulator which demodulates areceived signal to a digital signal, 108 designates an error-correctiondecoder which detects and corrects a transmission error, 113 designatesa buffer memory which is used for supplying compressed codes that havebeen received at a constant speed, in accordance with the request fromthe next stage, 111 designates a high-efficiency code decoder whichexpands the compressed video signal to the original signal, and 109designates a D/A converter which converts the digital video signal intoan analog video signal.

[0040] The redundancy of an input video signal always varies, andtherefore, the amount of codes which are compression-coded using thisredundancy also varies. However, the amount of information which can betransmitted through the transmission path 114 is limited. In order toexhibit the best of the performance, the variation of the code amount isbuffered using the buffer memory 113, and the information amount iscontrolled to be within a predetermined range so that overflow orunderflow of a memory does not occur. FIG. 24 shows the buffer operationperformed at the receiving side. Data which have been received at aconstant rate are stored in the buffer memory, and, when the data amountreaches the level B0, decoding of the codes starts. At the time whendata of d1 have been consumed for the display of the first picture andthe decoding for the second picture starts, the amount of theaccumulated data is B1. In the same manner, data accumulation and dataconsumption are alternately repeated. The amount of consumed data variesdepending on the displayed picture, but the average amount of consumeddata is equal to the receiving rate. The operation of the receiving sidehas been described. The operation of the transmitting side is performedin the entirely opposite way to that of the receiving side.

[0041] Since the communication apparatus is controlled as describedabove, the relationship between fields of an input video signal andtransmitted codes is not clearly defined. Unlike an application in thefield of communication, a VTR is required to perform functions peculiarto a VTR and including a special reproduction different from normalreproduction such as a still reproduction, slow reproduction andhigh-speed reproduction, an assemble edition, and an insert edition.Therefore, it is desirable to clearly define the relationship betweenfields and tracks. In order to produce a practical VTR, it is essentialto select a recording format which can solve these problems.

[0042] As a method of compressing a moving picture such as a televisionsignal, there is a method using an intra-field (or intra-frame) in whichthe encoding is completed within an individual field (or frame)independently of another field (or frame), and a prediction-field (orprediction frame) in which the predictive encoding is performed usinginformation of another field (or frame). Generally, the informationamount of the intra-field (or intra-frame) in which the predictionbetween fields (or frames) is not used is two or more times the codeamount of the prediction-field (or prediction frame) in which theencoding is performed using the prediction between planes. When recordareas of the same size (number of trucks) are allocated to theintra-field (or intra-frame) and the prediction-field (or predictionframe), therefore, there arises a fourth problem that in the intra-field(or intra-frame), the record area is not sufficient and in theprediction-field (or prediction frame), the record area has a uselessportion.

SUMMARY OF THE INVENTION

[0043] It is an object of the invention to provide a high-efficiencyencoder in which an intra-field (or intra-frame) that does not performthe motion compensation is formed every n fields (or n frames) andmotion-compensation prediction with an intra-field (or intra-frame) isperformed in other fields (or frames), thereby solving the first problemand reducing the hardware size.

[0044] It is another object of the invention to provide ahigh-efficiency encoder in which a reference picture is previously setusing a method fixed somewhat for a normal picture having no scenechange, and, when a scene change occurs in the step of themotion-compensation prediction process, the reference picture isswitched so as to suppress the amount of generated information as muchas possible, thereby solving the second problem and the S/N ratio of apicture can be maintained while suppressing the increase in the hardwaresize.

[0045] It is a further object of the invention to provide ahigh-efficiency encoder in which motion-compensation prediction isperformed while a reference picture is previously set using a methodfixed somewhat for a normal picture having no scene change, and, when ascene change occurs in the step of the motion-compensation predictionprocess, the picture immediately after the scene change is encoded in afield (or frame) as an intra-field (or intra-frame), thereby solving thethird problem and suppressing the deterioration of a picture qualityafter the scene change as much as possible.

[0046] It is a still further object of the invention to provide a videoinformation recording/reproducing apparatus in which signals of aplurality of fields (or frames) are collected into one recording unit tobe recorded in predetermined number of tracks, thereby solving thefourth problem and coping with the special reproduction and editionrequired in a VTR.

[0047] In a high-efficiency encoder of the invention, only anintra-field (or intra-frame) is used as a reference picture formotion-compensation prediction, and therefore the informationcompression in which the deterioration of the image quality is notconspicuous can be performed with a reasonable hardware size.

[0048] In the other high-efficiency encoder of the invention,motion-compensation prediction is usually performed using a fixedreference picture, and, when a scene change occurs in the step of themotion-compensation prediction process, the reference picture isadaptively switched, thereby suppressing the increase in the informationamount. In this case, for example, blocks which are judged to be in anintra mode are counted in order to determine whether a scene changeoccurred, and, when blocks of an intra mode, the number of which isgreater than a preset threshold, are generated, the reference picturefor the next field (or frame) is switched. Therefore, the amount ofgenerated codes can be suppressed and the S/N ratio of a reproducedpicture can be maintained at a high level, by monitoring the encodingmode of a block, determining whether a scene change occurred, andswitching the reference picture for motion-compensation prediction.

[0049] When a scene change is detected, the picture immediately afterthe scene change makes an intra picture and the information amount iscompressed, an encoding error is caused only by the difference in thisfield (or frame), thereby reducing the effect on the motion-compensationprediction process of the next field (or frame). Namely, whenmotion-compensation prediction is performed before and after a scenechange, the amount of information generated in the field (or frame) isincreased, resulting being that, even when the information iscompressed, the effect of an encoding error becomes greater. Therefore,when the same amount of information as that of the generated informationis encoded as an intra-field (or intra-frame), higher subjectiveevaluation can be obtained. In the further high-efficiency encoder ofthe invention, when a scene change occurs in a motion-compensationprediction process unit, the field (or frame) immediately after thescene change is encoded as an intra-field (or intra-frame). Accordingly,even when the information amount of the field (or frame) is compressedto the level of a prediction-field (or prediction frame), by performingencoding of the field as an intra-field (or intra-frame), thedeterioration of the image quality of a picture can be suppress moreefficiently.

[0050] In the video information recording/reproducing apparatus of theinvention, input signals of n fields (or n frames) are collected intoone recording unit block, and recorded in tracks of a predeterminednumber which is calculated from the amount of information to be recordedand the recording capacity of one track. The compression encoding isperformed on blocks which are collected into the recording unit in sucha manner that at least one intra-field (or intra-frame) is included inthe unit. Input television signals of n fields (or n frames) aresubjected as one recording unit block to the compression encoding by ahigh-efficiency encoder. The compression-encoded television signals of nfields (or n frames) are divided to be recorded in recording areas of mtracks. The reproduced signals of m tracks are restored to televisionsignals of n fields (or n frames) by a high-efficiency decoder.

[0051] The above and further objects and features of the invention willmore fully be apparent from the following detailed description withaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0052]FIG. 1 is a block diagram showing the configuration of a digitalVTR;

[0053]FIG. 2 is a block diagram showing the configuration of aconventional motion-compensation prediction processing apparatus;

[0054]FIG. 3 shows the block selection operation in the conventionalmotion-compensation prediction processing apparatus;

[0055]FIG. 4 is a diagram showing the relationship between fields in amotion-compensation prediction process;

[0056]FIG. 5 is a diagram showing the relationship between fields in amotion-compensation prediction process;

[0057]FIG. 6 shows code amounts and S/N ratios in conventionalmotion-compensation prediction;

[0058]FIG. 7 shows the change of the information amount for five secondsin case that a reference picture is not switched;

[0059]FIG. 8 shows the variation of the S/N ratio for five seconds inthe case that a reference picture is not switched;

[0060]FIG. 9 is a diagram showing the tape format according to the 8-mmVTR standard;

[0061]FIG. 10 is a diagram showing the format of one track according tothe 8-mm VTR standard;

[0062]FIG. 11 is a diagram showing the relationship between a rotationalhead drum and a magnetic tape wound around it used in an 8-mm VTR;

[0063]FIG. 12 is a graph showing the frequency allocation of each signalin the 8-mm VTR standard;

[0064]FIG. 13 is a block diagram showing the configuration of aconventional video information recording/reproducing apparatus;

[0065]FIG. 14 is a timing chart showing the relationship in phasebetween pulse signals for switching a head and input video signals inthe video information recording/reproducing apparatus of FIG. 13;

[0066]FIG. 15 is a waveform chart showing video signals processed bygate circuits in the video information recording/reproducing apparatusof FIG. 13;

[0067]FIG. 16 is a waveform chart showing time-base multiplexed signalsin the video information recording/reproducing apparatus of FIG. 13;

[0068]FIG. 17 is a block diagram showing the configuration of anotherconventional video information recording/reproducing apparatus;

[0069]FIG. 18 is a diagram showing the tape formats of VTRs of the D1and D2 methods;

[0070]FIG. 19 shows the overall specifications of VTRs of the D1 and D2methods;

[0071]FIG. 20 shows the specifications of the tape formats of VTRs ofthe D1 and D2 methods;

[0072]FIG. 21 shows the specifications of the tape running systems ofVTRs of the D1 and D2 methods;

[0073]FIG. 22 is a diagram showing the ranges of pixels recorded by VTRsof the D1 and D2 methods;

[0074]FIG. 23 is a block diagram showing the configuration of acommunication apparatus of a high-efficiency encoded video informationcompression method;

[0075]FIG. 24 illustrates the buffer operation of the high-efficiencycode communication apparatus;

[0076]FIG. 25 is a block diagram showing the configuration of ahigh-efficiency encoder according to the invention;

[0077]FIG. 26 is a diagram showing the relationship between fields in amotion-compensation prediction process;

[0078]FIG. 27 is a diagram showing the configuration of anotherhigh-efficiency encoder according to the invention;

[0079]FIG. 28 is a diagram showing the relationship between fields in amotion-compensation prediction process;

[0080]FIG. 29 shows simulation results obtained in the case that thereis a scene change;

[0081]FIG. 30 shows simulation results obtained in the case that thereis no scene change;

[0082]FIG. 31 is a block diagram showing the configuration of a furtherhigh-efficiency encoder according to the invention;

[0083]FIG. 32 is a flowchart of the operation of the high-efficiencyencoder of FIG. 31;

[0084]FIG. 33 is a flowchart of an intra-field process in FIG. 32;

[0085]FIG. 34 is a flowchart of a prediction-field process in FIG. 32;

[0086]FIG. 35 shows the change of the information amount for fiveseconds in the case that a reference picture is switched;

[0087]FIG. 36 shows the variation of the S/N ratio for five seconds incase that a reference picture is switched;

[0088]FIG. 37 is a flowchart of another prediction-field process in FIG.32;

[0089]FIG. 38 is a flowchart of a further prediction-field process inFIG. 32;

[0090]FIG. 39 is a flowchart of a reference picture switching judgingprocess in FIG. 38;

[0091]FIG. 40 is a block diagram showing the configuration of a stillfurther high-efficiency encoder according to the invention;

[0092]FIG. 41 shows the operation of selecting blocks in thehigh-efficiency encoder of FIG. 40;

[0093]FIG. 42 is a diagram showing a switching between a referencepicture and an intra-field in the high-efficiency encoder of FIG. 10;

[0094]FIG. 43 is a flowchart of an operation of the high-efficiencyencoder of FIG. 40;

[0095]FIG. 44 is a flowchart of a prediction-field process in FIG. 43;

[0096]FIG. 45 is a diagram showing another switching between a referencepicture and an intra-field in the high-efficiency encoder of FIG. 40;

[0097]FIG. 46 is a flowchart of another operation of the high-efficiencyencoder of FIG. 40;

[0098]FIG. 47 is a diagram showing the relationship between fields in amotion-compensation prediction process;

[0099]FIG. 48 is a diagram showing the relationship between fields in amotion-compensation prediction process;

[0100]FIG. 49 is a diagram showing the switching of a reference picture;

[0101]FIG. 50 is a block diagram showing the configuration of a videoinformation recording/reproducing apparatus according to the invention;

[0102]FIG. 51 is a diagram showing an example of the tape formataccording to the invention;

[0103]FIG. 52 is a block diagram showing the internal configuration ofthe high-efficiency encoder shown in FIG. 50;

[0104]FIG. 53 is a graph showing an example of the variation of theamount of generated data for each frame; and

[0105]FIG. 54 is a diagram illustrating the relationship betweenrecorded information in each field and the writing in tracks accordingto the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0106] Embodiment 1

[0107] Hereinafter, a first embodiment of the present invention will bedescribed with reference to the drawings. FIG. 25 is a block diagramillustrating the first embodiment of the invention. In FIG. 25, 1designates an input terminal for a digital video signal, 2 designates ablocking circuit which form into a block a digital video signal inputthrough the digital video signal input terminal 1, 30 designates amotion-compensation predicting circuit which performsmotion-compensation prediction between the block output from theblocking circuit 2 and an intra-field and outputs a difference signalbetween an input block and a prediction block, 31 designates adeterminer which selects the one having the smaller sum of absolutevalues between the input signal from the blocking circuit 2 and aprediction difference signal from the motion-compensation predictingcircuit 30, 32 designates a first switch which selectively outputsencoded blocks output from the blocking circuit 2 and determiner 31,depending on the determined mode, 33 designates a orthogonal transformcircuit which performs the orthogonal transform on an encoded blockoutput from the first switch 32, 34 designates a quantizing circuitwhich quantizes the output of the orthogonal transform circuit 33, 11 isa transmission path. 35 designates a second switch which selects andoutputs the quantizing results output from the quantizing circuit 34,only in the case of an intra-field, 36 designates an inverse quantizingcircuit which inverse-quantizes the output of second switch 35, 37designates an inverse orthogonal transform circuit which performs aninverse orthogonal transform on the output of the inverse quantizingcircuit 36, and 38 designates a video memory which stores one field ofthe reproduced image of an intra-field output from the inverseorthogonal transform circuit 37 and outputs a reference picture in asearch range of the prediction field to the motion-compensationpredicting circuit 30.

[0108] As a predictive method used in such a circuit block, for example,the method shown in FIG. 26 may be employed. In this method, anintra-field is inserted every four fields, and the three intermediatefields makes prediction-fields. In FIG. 26, first field F1 is anintra-field, and second, third and fourth fields F2, F3 and F4 areprediction-fields. In this method, second, third and fourth fields F2,F3 and F4 are predicted from first field F1. At first, first field F1which is an intra-field is blocked in the field, and the orthogonaltransform being performed, the block is quantized, and then encoded. Inthe local decoding loop, the quantized signals of first field F1 aredecoded or reconstructed. The reconstructed image is used inmotion-compensation prediction for second, third and fourth fields F2,F3 and F4. Then, motion-compensation prediction is performed on secondfield F2 using first field F1. After orthogonal transform is performedon the obtained difference blocks, encoding is performed in a similarmanner as in first field F1. In this case, when the sum of the absolutevalues of the input blocks is smaller than that of the differenceblocks, orthogonal transform is performed on the input blocks in placeof the difference blocks, and thereafter encoding is performed in asimilar manner as in first field F1. In a similar manner as in secondfield F2, using first field F1, also third and fourth fields F3 and F4are motion-compensation prediction processed and encoded. Also in thirdand fourth fields F3 and F4, when the sum of absolute values of the ACpowers of the input blocks is smaller than that of the differenceblocks, orthogonal transform is performed on the input blocks in placeof the difference blocks, and thereafter encoding is performed in asimilar manner as in first field F1.

[0109] Then, the operation will be described. Irrespective of the field(an intra-field or a prediction-field), digital video signals inputthrough the digital video signal input terminal 1 are blocked by theblocking circuit 2 in the unit of, for example, 8 pixels×8 lines. Themotion-compensation predicting circuit 30 performs motion-compensationprediction on input blocks which are output from the blocking circuit 2in the case of a prediction-field, while using as a reference picturethe reproduced video data of an intra-field stored in the video memory38. The motion-compensation predicting circuit 30 performs motionestimation while setting the search range of motion estimation at, forexample, 16 pixels×16 lines, to obtain a motion vector, and furtherobtains the difference signal between the reference picture and theinput image in accordance with the motion vector obtained in the motionestimation. The difference signal is output together with the motionvector to determiner 31. The determiner 31 obtains the sum of absolutevalues of components of each of the input blocks output from theblocking circuit 2 and the difference blocks output from themotion-compensation predicting circuit 30. When the input blocks areindicated by I(i, j) (i, j=1 to 8), the sum of their absolute values isindicated by Is, the difference blocks are indicated by P(i, j) (i, j=1to 8), and the sum of their absolute values is indicated by Ps, Is andPs can be expressed by the following expressions: $\begin{matrix}{{Is} = \left. {\sum\limits_{j = 1}^{8}\quad \sum\limits_{i = 1}^{8}}\quad \middle| {I\left( {i,\quad j} \right)}\quad \right|} \\{{Ps} = \left. {\sum\limits_{j = 1}^{8}\quad \sum\limits_{i = 1}^{8}}\quad \middle| {P\left( {i,\quad j} \right)} \right|}\end{matrix}$

[0110] When Ps<Is, it is determined that the information amount of thedifference blocks is smaller than that of the input signal blocks, andthen the difference blocks are output together with the motion vectorsto first switch 32. In contrast, when Ps≧Is, it is determined that theinformation amount of the input signal blocks is smaller than that ofthe difference signal blocks, and then the input blocks are outputtogether with a forced intra signal indicating that the block is theforced intra block, in place of the motion vectors, to the first switch32.

[0111] The first switch 32 selects the output of the blocking circuit 2in the case of an intra mode, and the output of the determiner 31 in thecase of a prediction mode, and supplies the selected output to theorthogonal transform circuit 33. The orthogonal transform circuit 33performs, for example, the two-dimensional DCT on each of the 8×8 blockswhich are input thereinto. The quantizing circuit 34 performs variablelength coding and quantizes the orthogonal transform coefficients outputfrom the orthogonal transform circuit 33. Furthermore, in the case of aprediction mode, quantizing circuit 31 quantizes also the motion vectorsor the forced intra signals in addition to the orthogonal transformcoefficients, and outputs them together to a transmission path 11. Onthe other hand, in order to produce reference data for themotion-compensation prediction, only in the case of the intra-field, thesecond switch 35 outputs the orthogonal transform coefficients whichhave been quantized by the quantizing circuit 34, to the inversequantizing circuit 36. The inverse quantizing circuit 36inverse-quantizes and performs variable length decoding the data whichhave been performed variable length coding by the quantizing circuit 34,and the decoded data are output to inverse orthogonal transform circuit37. The inverse orthogonal transform circuit 37 performs, for example,the inverse two-dimensional DCT on the data to reconstruct the blocks ofthe intra-field. The blocks of the intra-field which have beenreconstructed by the inverse orthogonal transform circuit 37 are storedin the video memory 38. The video memory 38 stores the reconstructedpictures of the intra-field for one field, as the reference data in thecase of the motion-compensation prediction, and outputs the referencepictures within the motion vector detection range to motion-compensationpredicting circuit 30.

[0112] In the embodiment described above, a block for the orthogonaltransform has the size of 8 pixels×8 lines. This size is not necessarily8 pixels×8 lines, and the block may have the size of n pixels×m lines.Similarly, it is not necessary to set the search range for motionvectors to 16 pixels×16 lines, and the search range may be set to kpixels×s lines (k≧n, s≧m). In the above embodiment, the predictiveencoding is completed every four fields. It is not necessary to completethe predictive encoding every four fields, and the predictive encodingmay be completed every arbitrary number of fields. Furthermore, it isnot necessary to complete the predictive encoding every arbitrary numberof fields, and predictive encoding may he completed every arbitrarynumber of frames. According to the above embodiment, in the determiner31, the output having the smaller sum of absolute values is selectedfrom the outputs of the blocking circuit 2 and motion-compensationpredicting circuit 30, to be output to the first switch 32.Alternatively, without performing the motion-compensation determination,only the output of the motion-compensation predicting circuit 30 may beoutput to the first switch 32.

[0113] Embodiment 2

[0114] According to Embodiment 1 described above, in the determiner 31,the output having the smaller sum of absolute values is selected fromthe outputs of the blocking circuit 2 and motion-compensation predictingcircuit 30, to be output to the first switch 32. In a field wheredeterminer 31 selects the forced intra mode more frequently than theprediction mode, it may be judged that a scene change has occurred inthe field, and assuming that the whole field is in the intra modeencoding may be performed. An embodiment constructed so as to performthis operation is Embodiment 2 described below.

[0115]FIG. 27 is a block diagram showing the configuration of the secondembodiment. In the figure, 40 designates a determiner which selects theone having the smaller sum of absolute values, from an input block fromthe blocking circuit 2 and a prediction difference block from themotion-compensation predicting circuit 30, and which judges a fieldwhere the input block from the blocking circuit 2 is selected morefrequently, to be an intra-field, 41 designates a first field memorywhich stores the input blocks output from the blocking circuit 2 as dataof an intra-field, 42 designates a second field memory which stores theblocks of a prediction-field output from the determiner 40, and 43designates a first switch which, in the intra mode and when thedeterminer 40 judges that the forced intra mode more frequently occursthan the prediction mode, selects the output of the first field memory41 and outputs it to the orthogonal transform circuit 33, and which, inother case than the above-mentioned two cases, selects the output of thesecond field memory 42.

[0116] Then, the operation will be described. In the process between thedigital video signal input terminal 1 and motion-compensation predictingcircuit 30, operation is carried out in the same manner as those in thefirst embodiment, and therefore the description is omitted. Thedeterminer 40 selects the one having the smaller sum of absolute valuesof components, from input blocks output from the blocking circuit 2 anddifference blocks output from the motion-compensation predicting circuit30, and outputs it. When the output from the motion-compensationpredicting circuit 30 is selected, the determiner 40 outputs motionvectors and blocks of difference signal. When the output from theblocking circuit 2 is selected, motion vectors are output together witha signal indicative of a forced intra-block. When the number of forcedintra-blocks in one field reaches or exceeds a value n, determiner 40judges that a scene change has occurred, and outputs a control signal sothat the whole current field is to be encoded by the intra mode.

[0117] The output of the determiner 40 is stored as data of theprediction mode in the second field memory 42, and, after data for onefield have been stored, it is output to the first switch 43. Incontrast, the output of the blocking circuit 2 is stored as data of theintra mode in the first field memory 41, and, after data for one fieldhave been stored, it is output to the first switch 43. In the intra modeand when the determiner 40 determines to force the intra mode on thefield, the first switch 43 selects the output of the first field memory41, and, in another case, selects the output of second field memory 42.The output of the first switch 43 is supplied to the orthogonaltransform circuit 33. In the succeeding process from the orthogonaltransform circuit 33 to the video memory 38, the operation is carriedout in the same manner as those in the first embodiment, and thereforethe description is omitted. However, when it is determined by thedeterminer 40 that a scene change occurs, it is required to update alsothe contents of the video memory 38. In this case, therefore, the secondswitch 35 operates, in the same manner as in the intra mode, to supplythe output of the quantizing circuit 34 to the inverse quantizingcircuit 36.

[0118] Hereinafter, examples in which encoding and decoding arerespectively performed by three kinds of predictive encoding methodsshown in FIGS. 26, 4 and 28 will be described. In the predictiveencoding method shown in FIG. 28, predictions between fields isperformed in a field, and third field F3 is predicted from first fieldF1. Referring to the encoding methods shown in FIGS. 26, 4 and 28respectively as methods 1, 2 and 3, FIG. 29 shows result of simulationprocesses which were conducted on sample images for five seconds whereina scene change exists. Similarly, FIG. 30 shows results of simulationprocesses which were conducted on sample images for five seconds whereinno scene change exists. For these sample images, 4:2:2 component signals(Y: 720×240, Cb and Cr: 360×240, 60 Fields/sec.) are used. As seen fromthe results shown in FIGS. 29 and 30, to images having a scene change,method 3 is advantageous in the view point of the S/N ratio, but, forimages having no scene change, there is little difference betweenmethods 1 to 3. As a result, when a scene change occurs, the intra-fieldis employed forcedly, and a high-efficiency encoder of which hardwaresize is smaller than that of a conventional predictive encoder can berealized.

[0119] In Embodiment 2 described above, the intra mode is created everyn fields and the succeeding (n−1) fields are prediction-encoded from anintra-field, and, when a scene change occurs, the intra-field isforcedly created and the remaining fields are predicted from the forcedintra mode. It is not necessary that an intra-field exists every nfields. Alternatively, when a forced intra-field is created, thesucceeding (n−1) fields which comes after the forced intra-field may beprediction-encoded. In the above-described embodiment, the predictiveencoding is performed in the unit of field. It is not necessary toperform predictive encoding in the unit of field, and predictiveencoding may be performed in the unit of frame.

[0120] As described above, since the high-efficiency encoders ofEmbodiments 1 and 2 create an intra-field every n fields and in theother fields, perform motion-compensation prediction using thisintra-field as a reference picture, the hardware size including acalculation circuit for obtaining motion vectors can be reduced.

[0121] Embodiment 3

[0122]FIG. 31 is a block diagram showing the configuration of ahigh-efficiency encoder in Embodiment 3. In FIG. 31, 1-14 and 16-20designate the elements identical to those of the conventional apparatusin FIG. 2. The reference numeral 50 designates a mode counter whichcounts the number of blocks of the intra mode, 51 designates a directionswitching circuit which compares a predetermined number of blocks withthe number of blocks of the intra mode output from the mode counter andwhich determines the reference picture for the next field, and 52designates a video memory which stores output blocks in order to performmotion-compensation prediction and which outputs the reference picturefor the next field as the search range.

[0123] Then, the operation will be described. Irrespective of anintra-field or a prediction-field, input digital video signals are bythe blocking circuit 2 segmented into input blocks a unit of whichconsists of m [pixels]×n [lines]. In order to obtain a difference block,subtracter 3 calculates the difference in the unit of pixel between aninput block and a prediction block. In this way, an input block and adifference block are supplied to first switch 7. In order to calculatethe power, the difference block is supplied also to difference powercalculator 4 and the difference power is calculated. In order tocalculate the AC power, the input block is supplied also to the originalpower calculator 5 and the original power is calculated. The outputs ofthe difference power calculator 4 and original power calculator 5 aresupplied to the determiner 6 which in turn selects the one having thesmaller power from the two powers outputting the result to the firstswitch 7 as the mode signal. More specifically, when the differencepower is smaller than the original AC power, the prediction mode signalis output so that the first switch 7 is set to the prediction mode tooutput the difference block as an encoded block. When the original poweris smaller than the difference power, the intra mode signal is output sothat first switch 7 is set to the intra mode to output the input blockas an encoded block.

[0124] The mode signal from determiner 6 is input to the mode counter50. Since the input mode signal is generated for every block of aprediction-field, the mode counter 50 counts the number of blocksselecting the intra mode or prediction mode, from the blocks of onefield, and outputs the number of blocks selecting an intra mode orprediction mode to the direction switching circuit 51. The directionswitching circuit 51 compares a predetermined number of blocks (which issmaller than the total number of blocks of one field) with the number ofblocks selecting an intra mode which number has been input from modecounter 50, and outputs a reference picture switching signal to videomemory 52. When the predetermined number of blocks is greater (orsmaller) than the number of blocks selecting an intra mode (or aprediction mode), direction switching circuit 51 outputs to video memory52 a reference picture switching signal so that the reference picture isnot to be switched. When the number of predetermined blocks is smaller(or greater) than the number of blocks selecting an intra mode (or aprediction mode), the direction switching circuit 51 outputs to thevideo memory 52 a reference picture switching signal so that thereference picture is to be switched.

[0125] The first switch 7 outputs an input block or a difference blockas an encoded block, depending on the mode signal determined by thedeterminer 6. In this case, when the input block is the intra-field, thefirst switch 7 surely outputs the input blocks as encoded blocks. Theencoded blocks enter the DCT circuit 8 to be converted into DCTcoefficients. The DCT coefficients are subjected to the weighting andthreshold processes in the quantizing circuit 9 to be quantized topredetermined bit numbers each corresponding to the coefficients. TheDCT coefficients which have been quantized to the respectivepredetermined bit numbers are converted by the first encoder 10 intocodes suitable for transmission path 11 and then output to transmissionpath 11.

[0126] The DCT coefficients on which weighting and threshold processesand quantization have been performed by the quantizing circuit 9 alsoenter the local decoding loop 20, and are subjected to the inverseweighting and inverse quantizing processes in the inverse quantizingcircuit 12. Then, the DCT coefficients on which inverse weighting andinverse quantizing processes have been performed in the local decodingloop 20 are converted into a decoded block by inverse DCT circuit 13.The adder 14 adds the decoded block to a prediction block in the unit ofpixel. This prediction block is the same as that used in the subtracter3. The result of addition by the adder 14 is written as an output blockin a predetermined address of the video memory 52. Video memory 52switches the reference picture in response to the reference pictureswitching signal from the direction switching circuit 51, and outputsthe motion estimation search range to the MC circuit 16. The size of theblock in the motion estimation search range is, for example, i[pixels]×j [lines] (where i≧m, j≧n). A block in the motion estimationsearch range output from the video memory 52, and an input block fromthe blocking circuit 2 are input into the MC circuit 16. The MC circuit16 performs the motion estimation on each block to extract motionvectors of the input block.

[0127] The motion vectors extracted by the motion estimation in the MCcircuit 16 are input into the MIX circuit 17. The MIX circuit 17combines the motion vectors from the MC circuit 16 with the mode signaldetermined by the determiner 6. The motion vectors and mode signal whichhave been combined with each other in the MIX circuit 17 are convertedby the second encoder 18 into codes suitable for the transmission path11, and then output together with the corresponding encoded blocks tothe transmission path 11. The prediction block is output from the MCcircuit 16 in the form of a block which is blocked into the size (m[pixels]×n [lines]) equal to that of the input block from the motionestimation range. This prediction block is supplied to the second switch19, and output from the respective output terminal of the switch, inaccordance with the field of the input block the currently beingprocessed and the mode signal of the decoded block. Namely, theprediction block is output from one of the output terminals of thesecond switch 19 to the subtracter 3 in accordance with the processedfield, and from the other output terminal in accordance with the modesignal of the current decoded block and the processed field.

[0128] According to the present invention, in the case that a scenechange occurs in the unit of frame when the predictive method shown inFIG. 4 is used for an ordinary picture, the number of blocks, whichselect an intra mode in encoding the picture immediately after the scenechange, increases, and the reference picture used thereafter can beswitched as shown in FIG. 28.

[0129] The operation in Embodiment 3 is summarized below with referenceto flowcharts in FIGS. 32, 33 and 34. FIG. 32 is a flowchart showing thewhole operation in Embodiment 3, FIG. 33 is a flowchart showing anintra-field process of step S103 in FIG. 32, and FIG. 34 is a flowchartshowing a prediction-field process of step S104 in FIG. 32.

[0130] Firstly, the field number fn indicative of the field in themotion-compensation prediction process unit is set to be 0 (step S101).Referring to FIG. 4, this field number fn will be described. Intra-fieldF1 which comes first in the motion-compensation prediction process unitis identified by the field number fn=0, intra-field F2 is identified bythe field number fn=1, next intra-field F3 is identified by the fieldnumber fn=2, and intra-field F4 which comes last in themotion-compensation prediction process unit is identified by fieldnumber fn=3. Since a motion-compensation process has been just started,the field to be initially processed is surely the first field in themotion-compensation prediction process unit and an intra-field, andtherefore the field number fn is set to be 0 (fn=0) in step S101. Areference picture switching flag Rfn which functions as a flag fordetermining whether a scene change exists is set in a laterprediction-field process, but in this step the flag Rfn is set to be 0(Rfn=0) for initialization.

[0131] Then, the field number fn is checked to determine whether it is 0or whether the field is the first field in the motion-compensationprediction process unit and an intra-field (step S102). If the fieldnumber fn is 0 (fn=0), this field is processed as an intra-field (stepS103). In contrast, if the field number fn is not 0 (fn≠0), this fieldis processed as a prediction-field. These processes will be described indetail later. After each field is processed, the field number fn isincremented so as to indicate the next field (step S105). In an actualhardware, such a field number can be controlled by a microcomputersignal or the like.

[0132] It is judged whether the field number fn indicating the nextfield is a number indicating a field within the motion-compensationprediction process unit (step S106). If the field number fn is a numbernot indicating a field within the motion-compensation prediction processunit, for example, as in FIG. 4, wherein since the motion-compensationprocess unit has been completed within four fields and the field numberfn of an intra-field has been set to be 0, if fn=4, it means that aseries of motion-compensation prediction units have been completed. Iffn<4, it is judged that the next field is still within themotion-compensation process unit, and the process is repeated.

[0133] When a series of the motion-compensation prediction process unitshave been completed, it is judged whether all of the required fieldshave been processed (step S107). This judgment can be done by, forexample, checking the operation of an end switch of the high-efficiencyencoder. If the next field is to be processed, in order to encode thenext motion-compensation prediction process unit, the variables areinitialized, and the process is repeated. If the operation of thehigh-efficiency encoder has been completed, the encoding is ended.

[0134] The intra-field process will be described with reference to theflowchart in FIG. 33. The field which is determined in step S102 in FIG.32 to be processed as an intra-field is segmented into the predeterminedsize of m [pixels]×n [lines] in the processed field (step S201). Then,an orthogonal transform such as DCT is performed on the blocks of thatsize (step S202). The data on which orthogonal transform has beenperformed are quantized into a predetermined bit number which is set foreach sequence (step S203). In an orthogonal transform such as DCT,generally, a quantization is performed in such a manner that a largerbit number is assigned to a DC and low-order segments of AC, and asmaller bit number is assigned to a high-order sequence of AC. Thequantized data are converted into codes suitable for transmission (stepS204), and the encoded data are transmitted (step S205). It is judgedby, for example, counting the number of processed blocks, whether theprocess of one field has been completed (step S206). If the process ofone field has not yet been completed, the process for the next block ispursued. If the process of one field has been completed, the intra-fieldprocess is ended.

[0135] The prediction-field process will be described with reference tothe flowchart in FIG. 34. The field which is determined in step S102 inFIG. 32 to be processed as a prediction-field is checked to judgewhether Rfn−1=0, in the reference picture switch flag, in the process ofthe previous field or whether a scene change has been detected in theprocess of the field preceding the field currently being processed (stepS301). If Rfn−1=0, motion-compensation prediction is performed using thereference picture in the same position as before (step S302). IfRfn−1=1, it means that a scene change has been detected when fieldnumber fn−1 being processed. Therefore, in the motion-compensationprediction for field number fn the reference picture is switched to apicture of a field in a position different from the previous position,and motion-compensation prediction is performed using the new referencepicture (step S303).

[0136] Then, a variable COUNT for counting the number of blocks whichselect an intra mode in one field to be processed is set to be 0 (stepS304). The variable COUNT will be described in detail later. An inputpicture is segmented into the predetermined size of m [pixels]×n [lines]in the processed field (step S305). The blocks segmented into the sizeof m×n are subjected to motion-compensation prediction (step S306).Using the reference picture set in step S302 or S303, the difference inthe unit of pixel between a predetermined area of a past image and anewly divided block is input as a difference block into difference powercalculator 4, and a difference power P1 is calculated (step S307).Namely, by using such preset reference picture, the information amountgenerated by motion-compensation prediction can be reduced. Then, thenewly segmented block is input into original power calculator 5 tocalculate an original AC power P2 (step S308).

[0137] The calculated powers P1 and P2 are compared in magnitude witheach other (step S309). When the difference power P1 is smaller than theoriginal AC power P2, the difference block (the difference value of theblock subjected to motion-compensation prediction) is selected (stepS310). When the difference power P1 is greater than the original ACpower P2, the input block (the original which is still in block form isselected (step S311), and the number of times when an input block isselected as an encoded block or the number of blocks in one field whichare to be processed as an intra mode is counted (step S312). A variablethat functions as a counter in this step is the COUNT which has been setto be 0 in step S304. This counter is surely set to be 0 when theprocess in the unit of field starts, and counts the number of blockswhich select an intra mode in the processing of one field.

[0138] Each selected block is subjected to the orthogonal transform(step S313), and quantized to a predetermined bit number which is setfor each sequence (step S314). In an orthogonal transform such as DCT, aquantization is performed in such a manner that a larger bit number isassigned to a DC and low-order sequences of AC, and a smaller bit numberis assigned to a high-order sequence of AC. The quantized data areconverted into codes suitable for transmission (step S315), and theencoded data are transmitted (step S316). For example, the number ofprocessed blocks is counted to judge whether the process of one fieldhas been completed (step S317). If the process of one field has not yetbeen completed, the process for the next block is pursued.

[0139] If the process of one field has been completed, the number ofinput blocks which have been processed as encoded blocks in the processof that one field or the number of blocks which have selected an intramode is compared with a preset threshold TH (step S318). The thresholdTH is a predetermined number which is less than the number of blocks inone field. When the total number of blocks in one field is 2,700, forexample, the threshold TH is set to be 1,000 which is less than 2,700.If the variable COUNT indicating the number times when input blocksbeing selected as encoded blocks is smaller than the threshold TH, thereis no scene change between the field (field number fn) which has beenjust processed and the reference picture which has been used inmotion-compensation prediction of that field, and the reference pictureswitch flag Rfn is set to be 0 (Rfn=0) so that the reference picture ina normal position is used as the reference picture for themotion-compensation of the next field (field number fn+1) (step S319).If the variable COUNT indicating the number of times when input blocksbeing selected as encoded blocks is greater than the threshold TH, thereis a scene change between the field (field number fn) which has beenjust processed and the reference picture which has been used inmotion-compensation prediction of that field, and the reference picturefor the motion-compensation of the next field (field number fn+t) isswitched from the reference picture in a normal position to a field in aposition different from the position taken till then, for example, thefield which has been just processed and is positioned in a place where areference picture has not existed till then. For that purpose, thereference picture switch flag Rfn is set to be 1 (Rfn=1) (step S320). Inthis way, the reference picture switch flag Rfn is set and theprediction-field process is ended.

[0140]FIG. 35 shows the change of the information amount for fiveseconds in case that the predictive encoding is performed according tothe Embodiment 3, and FIG. 36 shows the variation of the S/N ratio forfive seconds in this case. Although a scene change exists at point B,the increase of information amount is suppressed as compared with pointA shown in FIG. 7. Furthermore, there is no conspicuous deterioration ofthe S/N ratio.

[0141] Embodiments 4 and 5

[0142] In Embodiment 3, in order to select an encoded block from adifference block and an input block, their powers are calculated andcompared with each other, and the number of blocks selecting an intramode are counted.

[0143] According to Embodiment 4, in order to select an encoded blockfrom a difference block and an input block, the entropy in each block iscalculated, and the entropy of the difference block is compared withthat of the input block in the same manner as in Embodiment 3 by thedeterminer 6, to determine which block is to be selected as an encodedblock.

[0144] According to Embodiment 5, in order to select an encoded blockfrom a difference block and an input block, adding of absolute values ofpixels is performed in each block, the rth power of the sum of absolutevalues of the input block and that of difference block are calculated,and the rth power of the sum of absolute values of the difference blockis compared with that of the input block in the same manner as inEmbodiment 3 by the determiner 6, to determine which block is to beselected as an encoded block.

[0145] Embodiment 6

[0146] In Embodiment 3, the determiner 6 compares the power of an inputblock with that of a difference block. According to Embodiment 6, whenthe power of an input block is to be compared with that of a differenceblock, at least one of the powers of the input and difference blocks isprovided with an offset, and then the two powers are compared with eachother. For example, the power of the input block is provided with apositive offset, and then compared with the power of the differenceblock. When there is not a great difference in power between the inputand difference blocks, this configuration allows the number of blocksselecting the difference power to be increased, thereby preventing anintra mode from being excessively created.

[0147]FIG. 37 is a flowchart of a prediction-field process in Embodiment6. In FIG. 37, portions designated by the same step numbers used in FIG.34 are identical with those of FIG. 34. The processes from step S301 tostep S308 are the same as in Embodiment 3. A difference power P1calculated from a difference block is compared with a value which isobtained by adding a predetermined offset a to an original AC power P2calculated from an input block (original block) (step S330). This makesit difficult to obtain P1<P2+α compared with Embodiment 3, so that thenumber of blocks selecting an intra mode is reduced. As a result, anintra mode is prevented from being excessively created, and thegenerated information amount can be stably kept at a constant level. Thesucceeding processes from step S310 to step S320 are the same as inEmbodiment 3.

[0148] Embodiments 7 and 8

[0149] According to Embodiment 7, when the entropy of a difference blockis compared with that of an input block in a similar manner as inEmbodiment 4, at least one of the entropy of the input and the entropyof the difference block is provided with an offset, and then the twovalues are compared with each other. For example, the entropy of theinput block is provided with a positive offset, and then compared withthe entropy of the difference block. When there is not a greatdifference between the entropy of the input block and the entropy ofdifference block, this configuration allows the number of blocksselecting difference power to be increased, thereby preventing an intramode from being excessively created.

[0150] According to Embodiment 8, when the sum of absolute values of adifference block is compared with that of an input block in a similarmanner as in Embodiment 5, at least one of the rth power of the sum ofabsolute values of the input block and that of the difference block isprovided with an offset, and then the two values are compared with eachother. For example, the rth power of the sum of absolute values of theinput block is provided with a positive offset, and then compared withthe rth power of the sum of absolute values of the difference block.When there is not a difference greater than the offset between the rthpower of the sum of absolute values of the input and that of thedifference block, this configuration allows the number of blocksselecting the difference power to be increased, thereby preventing anintra mode from being excessively created.

[0151] Embodiment 9

[0152] In Embodiment 3, the mode counter 50 counts the number of allblocks selecting an intra mode among blocks for one field. In Embodiment9, blocks for one field are not counted, but, at the time a mode signalin a predetermined number of blocks during one field is determined, theratio of the number of blocks selecting an intra mode to the totalnumber of blocks or the number of the blocks in which a mode signal hasbeen determined is supplied to the direction switching circuit 51. Basedon this ratio, a reference picture switching signal is output from thedirection switching circuit 51. This configuration allows the referencepicture for the next field to be determined even when encoding of allblocks for one field has not yet completed.

[0153]FIG. 38 is a flowchart of a prediction-field process in Embodiment9. In FIG. 38, portions designated by the same step numbers used in FIG.34 are identical with those of FIG. 34. The processes from step S301 tostep S303 are the same as in Embodiment 3. After the reference picturefor motion-compensation prediction for the next field is set (steps S302and S303), the variable COUNT for counting the times when intra modesbeing generated in one field during the processing of the field or thenumber of blocks selecting input blocks as encoded blocks, and variableB for counting the number of blocks which have been processed in theprocessing of the one field till then are set to be 0 (step S340). Thesucceeding steps S305 to S316 are the same as those of in Embodiment 3.After encoding, the number of block which have been processed till thenis counted by incrementing the variable B one by one (step S341). Thevariable B changes front 0 to the maximum number of blocks which canexist in one field. The reference picture switching determinationprocessing for determining whether the reference picture formotion-compensation prediction of the next field is to be switched isperformed (step S342). The next step S317 is the same as that ofEmbodiment 3.

[0154]FIG. 39 is a flowchart of the reference picture switchdetermination process in step S342 in FIG. 38. The process will bedescribed with reference to FIG. 39. It is judged whether the referencepicture switch flag Rfn is 0 (step S351). If the flag Rfn is not 0, theprocess is ended if the flag Rfn is 0, the ratio of the COUNT forcounting the times when input blocks being as encoded blocks to thevariable B for counting blocks which have been processed in theprocessing of the one field till then is compared with the threshold TH(step S352). If the ratio is smaller than the threshold TH, the processis ended. If the ratio is greater than the threshold TH, the flag Rfn isset to be 1 (step S353), and the process is ended.

[0155] Embodiment 10

[0156] Embodiment 10 will be described with reference to FIG. 40 whichshows the configuration of the embodiment. In FIG. 40, the referencenumerals 1, 3 to 6, 8 to 16, 18 and 20 are the same as those used in theconventional apparatus in FIG. 2. The reference numeral 60 designates avideo memory in which input pictures are stored, 61 designates an SCdetection circuit which detects a scene change in a picture and outputsa signal indicative of this, 62 designates a first switch which switchesfrom an input block segmented from an original picture to a differenceblock generated from a prediction block due to motion-compensationprediction, 63 designates a MIX circuit in which a motion vector, themode signal of a block from the determiner 6 and the scene change (SC)detection signal from the SC detection circuit 61 are combined, and 64designates a second switch which switches a prediction block.

[0157] Then, the operation will be described. It is assumed thatmotion-compensation prediction is performed, for example, as shown inFIG. 4 and is completed within four fields. Digital video signals inputthrough the input terminal 1 are stored in the video memory 60. Thevideo memory 60 has a memory for at least two fields, and, while storingvideo signals of one of the two fields, blocks video data for scenechange detection or processing into a predetermined size are output fromthe other field. Namely, the video memory 60 firstly sends digital videosignals to SC detection circuit 61, and characteristics of a picture,for example, obtained from preset parameters, and the presence of ascene change is detected. Then, digital video signals are output fromone of the outputs of the video memory 60 while being blocked into thesize of, for example, m [pixels]×n [lines] (where m and n are positiveintegers). The size of m [pixels]×n [lines] corresponds to the blocksize for performing the two-dimensional orthogonal transform, and alsoto the block size of a prediction block based on the motion-compensationprediction.

[0158] An input block which is obtained only by blocking an originaloutput from the video memory 60, and a difference block that is adifference between the input block and a prediction block which has beensubjected to motion-compensation prediction by the subtracter 3 areinput to the first switch 62. The input block and the difference blockare respectively input to the original power calculator 5 and thedifference power calculator 4 in order to obtain the power of eachblock. The original power calculator 5 calculates the AC power of theinput block, and the difference power calculator 4 calculates the powerof the difference block. The calculated AC power of the input block andthe calculated power of the difference block are input to the determiner6. When the power of the difference block is smaller than that of theinput block, the determiner 6 outputs a prediction mode signal, and,when the power of the input block is smaller than that of the differenceblock, the determiner 6 outputs an intra mode signal. These signals aresupplied as a mode signals to the first switch 62, the MIX circuit 63and the second switch 64.

[0159] The first switch 62 to which the input and difference blocks areinput outputs either of the blocks as an encoded block. For thatpurpose, the first switch 62 receives the scene change detection signalfrom the SC detection circuit 61 and also the mode signal from thedeterminer 6, to determine the switch mode, and outputs either of theinput and difference blocks as an encoded block. The switching states atthis time are shown in FIG. 41. Since the process step ofmotion-compensation prediction completes within four fields as shown inFIG. 4, in the ordinary mode an intra-field is the first field, aprediction-field then continues from the second field to the fourthfield, an intra-field is again the first field, and the above isrepeated continuously. With respect to the detection of the presence andabsence of a scene change shown in FIG. 41, when the scene changedetection signal from the SC detection circuit 61 indicates thedetection of a scene change, a signal of the presence is output, and,when the scene change detection signal does not indicate the detectionof a scene change, a signal of the absence is output. The discriminantmode means the mode signal which is an output of the determiner 6 anddescribed above. The symbol “X” in FIG. 41 means that the state is notaffected irrespective of the detection of a scene change or thediscriminant mode. As shown in FIG. 41, the first switch 62 determines aselection block, and outputs the selection block as an encoded block.

[0160] The encoded block which has been selected and output by the firstswitch 62 is subjected to two-dimensional orthogonal transform by theDCT circuit 8. The orthogonal-transformed data is subjected to theweighting and threshold processes or the like in the quantizing circuit9 to be quantized to a predetermined bit number in the respectivesequence. The data quantized by the quantizing circuit 9 are convertedby the first encoder 10 into codes suitable for the transmission path 11and then output to the transmission path 11. The data quantized by thequantizing circuit 9 are input also to the local decoding loop 20 sothat motion-compensation prediction is performed. The data input to thelocal decoding loop 20 are subjected to inverse quantizing and inverseweighting processes in inverse quantizing circuit 12, and then subjectedto the inverse orthogonal transform by the inverse DCT circuit 13. Adecoded block which is an output of inverse DCT circuit 13 is added inthe unit of pixel to the prediction block in the adder 14 to become areproduced image. The prediction block used in this process is identicalwith that used in the subtracter 3. The block which has become areproduced image in the adder 14 is written in a predetermined addressof the video memory 15.

[0161] The memory size of the video memory 15 depends on the type of theemployed predictive method. In this embodiment, it is assumed that thevideo memory 15 consists of a plurality of field memories, and thatoutput blocks reconstructed by the local decoding loop 20 are stored ina predetermined address. These stored images are used as data of thesearch range for motion-compensation prediction. A block which issegmented from an image reconstructed from past output blocks and is ina motion estimation search range is output from the video memory 15 tothe MC circuit 16. The size of the block of the motion estimation searchrange is i [pixels]×j [lines] (where i≧m, j≧n, and i and j are positiveintegers). Data in the search range for motion-compensation predictionfrom video memory 15 and an input block from video memory 60 are inputto the MC circuit 16 as reference data, thereby motion vectors beingextracted.

[0162] The motion vectors extracted by the MC circuit 16 are input tothe MIX circuit 63, and combined therein with the mode signal determinedby the determiner 6 and the SC detection signal from the SC detectioncircuit 61. The combined signals are converted by the second encoder 18into codes suitable for the transmission path 11, and then outputtogether with the corresponding encoded block to the transmission path11. The MC circuit 16 outputs blocked signals which are segmented fromthe search range in the size (m [pixels]×n [lines]) equal to that of theinput block, as a prediction block. The prediction block output from theMC circuit 16 is produced from past video information. The predictionblock is supplied to the second switch 64, and output in accordance withthe currently processed field, the mode signal of the decoded block andthe SC detection signal from the SC detection circuit 61. Namely, theprediction block is output from one of the output terminals of thesecond switch 64 to the subtracter 3 in accordance with the processedfield and the SC detection signal, and from the other output terminal inaccordance with the mode signal of the current decoded block, the SCdetection signal and the processed field.

[0163] The motion-compensation prediction process is shown in FIG. 42.In FIG. 42, it is assumed that a scene change occurs between secondfield F2 and third field F3. Since there is no scene change betweenfirst field F1 and second field F2, second field F2 is predicted fromfirst field F1. The scene change between second and third fields F2 andF3 is detected, and third field F3 becomes an intra-field in the samemanner as first field F1. Then, fourth field F4 is predicted from thirdfield F3. The prediction is never made on the basis of an image whichexists before that scene change. After the motion-compensationprediction process for fourth field F4 is completed, motion-compensationprediction is again performed while using the next field as anintra-field. Therefore, an intra-field surely appears every four fieldsonce the motion-compensation prediction process starts, and, when ascene change occurs, an intra-field exists also in themotion-compensation process.

[0164] The operation in Embodiment 10 will be summarized with referenceto flowcharts of FIGS. 43 and 44. FIG. 43 is a flowchart showing thewhole operation in Embodiment 10, and FIG. 44 is a flowchart of theprediction-field process in step S406 in FIG. 43.

[0165] At first, the field number indicative of a field in amotion-compensation prediction process unit is set to be 0 (step S401).This setting of the field number is the same as that in Embodiment 3.Since the motion-compensation process has been just started, the fieldto be initially processed is surely the first field in themotion-compensation prediction process unit and an intra-field, andtherefore the field number fn is set in step S401 to be 0. A scenechange detection flag Cfn which functions as a flag for judging whethera scene change is present is set in step S401 to be 0 for theinitialization.

[0166] Then, the characteristics of an input image are compared withthose of a past image by a certain parameter to detect the presence of ascene change (step S402). For example, the presence of a scene change isdetected by comparing the variance of values of pixels in somepredetermined areas of the past image with the variance of values ofpixels in some predetermined areas of the currently processed image.When a scene change is detected, the scene change detection flag Cfn isset to be 1 (Cfn=1), and, when a scene change is not detected, the scenechange detection flag Cfn is set to be 0 (Cfn=0).

[0167] Then, the field number fn is checked to judge whether it is 0 orthe field is the first field in the motion-compensation predictionprocess unit and an intra-field (step S403). If the field number fn is 0(fn=0), this field is processed as an intra-field (step S405). Incontrast, if the field number fn is not 0 (fn≠0), the process proceedsto next step S404. It is judged whether the scene change detection flagCfn is 0 (Cfn=0) or there is a scene change between the processed fieldand the reference picture required for encoding the field withmotion-compensation prediction (step S404). If the flag Cfn is 0(Cfn=0), there is no scene change, and the field to be processed isprocessed as a prediction-field (step S406). If the flag Cfn is 1(Cfn=1), there exists a scene change, and therefore the field to beprocessed is processed as an infrafield (step S405). Therefore, even inthe case that a field is in the motion-compensation prediction processunit and the field number is not 0, when a scene change is detected andthe flag Cfn is 1, the field is processed as an intra-field.

[0168] After each field is processed, the field number fn is incrementedso as to indicate the next field (step S407). In an actual hardware,such a field number can be controlled by a signal from a microcomputeror the like.

[0169] It is then judged whether the field number fn indicating the nextfield is a number indicating a field within the motion-compensationprediction process unit (step S408). If the field number fn is a numbernot indicating a field within the motion-compensation prediction processunit, for example, in FIG. 4 (if fn=4, FIG. 4), it indicates that aseries of motion-compensation prediction process units have beencompleted because, the motion-compensation prediction process unit hasbeen completed within four fields and the field number fn of anintra-field has been set to be 0. if fn<4, it is judged that the nextfield is within the motion-compensation prediction process unit, and theprocess is restarted from the scene change detection for the next fieldprocess. When the motion-compensation prediction process unit has beencompleted, it is judged whether all of the required fields have beenprocessed (step S409). This judgment can be done by, for example,checking the operation of an end switch of the high-efficiency encoder.If the next field is to be processed, in order to encode the nextmotion-compensation prediction process unit, the variables areinitialized, and the process is restarted from the scene changedetection. If the operation of the high-efficiency encoder is completed,is ended.

[0170] Next, the prediction-field process (step S406 in FIG. 43) inEmbodiment 10 will be described with reference to the flowchart of FIG.44. The field which has been determined in step S404 in FIG. 43 to beprocessed as a prediction-field is blocked into a predetermined size ofm [pixels]×n [lines] in the processed field (step S451). The blockssegmented into the size of m×n are subjected to motion-compensationprediction (step S452). The difference power P1 is calculated from adifference block which is the difference in the unit of pixel between apredetermined area of a past image and the block which has been justsegmented (step S453). The original AC power P2 kept in the state of theblock is calculated (step S454).

[0171] The calculated powers P1 and P2 are compared in magnitude witheach other (step S455). When the difference power P1 is smaller than theoriginal AC power P2, the difference block (the difference value of theblock subjected to motion-compensation prediction) is selected (stepS456). When the difference power P1 is greater than the original ACpower P2, the input block (the original which has been blocked and notsubjected to any further process) is selected (step S457). Each selectedblock is subjected to the orthogonal transform (step S458), and thequantized to a predetermined bit number which is set for each sequence(step S459). In an orthogonal transform such as DCT, for example, aquantization is performed in such a manner that a larger bit number isassigned to a DC and low-order sequences of AC, and a smaller bit numberis assigned to a high-order sequence of AC. The quantized data areconverted into codes suitable for transmission (step S460), and theencoded data are transmitted (step S461). It is judged by, for example,counting the number of processed blocks whether the process of one fieldhas been completed (step S462). If the process of one field has not yetbeen completed, the process for the next block is pursued. If theprocess of one field has been completed, the prediction field process isended.

[0172] According to Embodiment 10, when a scene change occurs in amotion-compensation prediction step as shown in FIG. 42, the fieldimmediately after the scene change is set to be an intra-field, wherebya subjective appreciation of the image immediately after the scenechange can be improved.

[0173] Embodiment 11

[0174] In Embodiment 10, even when a scene change occurs in a step of amotion-compensation prediction process and the field immediately afterthe scene change is set to be an intra-field, the time-constraint lengthin the motion-compensation prediction processing step is fixed for fourfields. Namely, an intra-field surely appears every four fields once themotion-compensation prediction process starts, and, when a scene changeoccurs, an intra-field exists also in the motion-compensation processingstep. This is a configuration in which a prediction-field is replacedwith an intra-field.

[0175] According to Embodiment 11, when a scene change occurs as shownin FIG. 45 and the field immediately after the scene change is set to bean intra-field, the intra-field is set to be the first field in themotion-compensation prediction process unit. That is, thetime-constraint length in the motion-compensation prediction step isvariable. Usually, the time-constraint length in the motion-compensationprediction step is set for four fields as shown in FIG. 45. When a scenechange occurs in the motion-compensation prediction step, the fieldimmediately after the scene change is set to be a new intra-field, andmotion-compensation prediction is performed in the unit of four fieldsbeginning with that field. When a scene change occurs in thismotion-compensation prediction step, the field immediately after thisscene change is set in a similar manner to be a new intra-field, andmotion-compensation prediction is performed in the unit of four fieldsbeginning with that field.

[0176]FIG. 46 is a flowchart showing the whole operation in Embodiment11. In FIG. 46, portions designated by the same step numbers used inFIG. 43 are identical with those in FIG. 43. The processes from stepS401 to step S406 are the same as in Embodiment 10. The intra-fieldprocess and prediction-field process in steps S405 and S406 are the sameas in Embodiment 10. With respect to a field which has been processed asan intra-field in step S405, the field number fn is set to be 0 (fn=0)in order to switch to the motion-compensation prediction process unit inwhich the first field is that processed field (step S490). In Embodiment10, even when a field is processed as an intra-field, for example, thefield number fn is sequentially changed in the order of 0→1→2→3→0 → . .. as shown in FIG. 45. In Embodiment 11, when a field which is not thefirst field in the motion-compensation prediction process unit isprocessed as an intra-field, the field number fn of that field isforcedly set to be 0, and that field is set to be the first field of thenew motion-compensation prediction process unit. This allows thetime-constraint length in the motion-compensation prediction processunit to be variable. When scene changes occur at a frequency shorter interms of time than the time-constraint length in the motion-compensationprediction process unit which is set at that time, the time-constraintlength in the motion-compensation prediction process unit becomes aseries of short lengths. The processes from step S407 to step S409 arethe same as in Embodiment 10.

[0177] According to Embodiment 11, the image immediately after the scenechange is set to be an intra-field, thereby improving a subjectiveappreciation of the image. When the frequency of the occurrence of scenechanges is longer than the time-constraint length in themotion-compensation prediction process unit and is low in level, thenumber of fields of an intra-field is smaller than that in Embodiment 3so that the information amount can be reduced.

[0178] Embodiment 12

[0179] In Embodiments 10 and 11, a process is performed while setting afield (or frame), in which a scene change is detected, to be anintra-field (or intra-frame). Alternatively, without setting as anintra-field (or intra-frame), the reference picture of the field (orframe) may be set to be intra-field (or intra-frame) belonging to thenext motion-compensation prediction process unit.

[0180] Embodiment 12 will be described with reference to FIG. 47. FIG.47(a) shows a usual motion-compensation prediction process which isperformed by the method in FIG. 4. In this example, fields F10 and F14function as an intra-field. The motion-compensation prediction isperformed while setting these fields F10 and F14 to be the first fieldof the motion-compensation prediction process unit. Then, when a scenechange occurs between field F11 and field F12 as shown in FIG. 47(b) andthe scene change is detected in field F12, fields from field F12 to thelast field (in this example, field F13) of the motion-compensationprediction process unit including field F12 are combined with the nextmotion-compensation prediction process unit, and fields F12 and F13 aresubjected to motion-compensation prediction in which an intra-fieldbelonging to the next motion-compensation prediction process unit isused as the reference picture. In the next motion-compensationprediction process unit combined with those fields, a normalmotion-compensation prediction and the motion compensation predictionfor the combined field as above are performed.

[0181] Embodiment 13

[0182] In Embodiment 12, the motion-compensation prediction process unitis sometimes longer than the usual one. According to Embodiment 13, Pfields (or P frames) which begin with a field (or frame) wherein a scenechange is detected and have the total length corresponding to the lengthof a usual motion-compensation prediction process unit are subjected tomotion-compensation prediction.

[0183] Embodiment 13 will be described with reference to FIG. 48. FIG.48(a) shows a usual motion-compensation prediction process which isperformed by the method in FIG. 4. In this example, fields F10 and F14function as an intra-field. While setting these fields F10 and F14 to bethe first field of a motion-compensation prediction process unit,motion-compensation prediction is performed. Then, when a scene changeoccurs between field F11 and field F12 as shown in FIG. 48(b) and thescene change is detected in field F12, a series of four fields (this isbecause a motion-compensation prediction process unit consists of fourfields) which begins with field F12 wherein the scene change is detectedare formed into a motion-compensation prediction process unit. Field F14which, in a usual case, may be the first field of the nextmotion-compensation prediction process unit and an intra-field is set tobe an intra-field in the current motion-compensation prediction processunit, and then motion-compensation prediction is performed. Whenmotion-compensation prediction for four fields beginning with field F12,or that for fields F12 to F15 is completed, the usualmotion-compensation prediction restarts at field F16 as it was before.

[0184] Embodiment 14

[0185] In Embodiments 3 to 13, a switching is explained with referenceto a scene change. A reference picture may be switched in accordancewith the number of intra modes which have been forcedly generated inblocks. Accordingly even in a picture wherein many forcible intra modesappear, or in a case that an object which has not existed in one fieldbefore appears suddenly in the current field or an object which hasexisted in one field before disappears suddenly from the current field,a reference picture can be switched by a similar method.

[0186] Embodiment 15

[0187] In Embodiments 3 to 13, for example, a process in whichmotion-compensation prediction as shown in FIG. 4 is performed isswitched to a process in which motion-compensation prediction as shownin FIG. 28 is performed. Before the switching, motion-compensationprediction of any kind may be performed as shown in FIG. 49. After thedetection of a scene change or the like, the process is switched tomotion-compensation prediction in which the generated information amountis reduced to a level lower than that before the switching as shown inFIG. 28.

[0188] Embodiment 16

[0189] In Embodiments 3 to 15, the motion-compensation predictionprocess is performed in the unit of four fields. The number of fieldswhich may be used as the unit is not necessarily four. The process maybe performed in the unit of an arbitrary number of fields on which themotion-compensation prediction process can be performed.

[0190] According to Embodiments 3 to 16, without largely increasing thememory amount by additionally providing a hardware as described above,even when a scene change occurs in the motion-compensation predictionprocess unit, a reference picture is switched from the originally setreference picture so as to minimize the influence caused by the scenechange, etc., the image immediately after the scene change is set to bea reference picture for motion-compensation prediction, and, after thedetection of the scene change, fields before the scene change are notused as the reference picture for motion-compensation prediction,whereby transmission can be done while suppressing the increase of thecode amount due to the scene change to a minimum and withoutdeteriorating the image quality.

[0191] When motion-compensation prediction is performed before and aftera scene change, the information amount of the predicted picture isusually increased. Therefore, by processing the field as an intrapicture with an information amount equal to that information amount, asubjective appreciation of the picture can be improved. While setting apicture immediately after a scene change as an intra picture bydetecting the scene change, encoding between fields or frames can beperformed so that a subjective appreciation of the picture immediatelyafter the scene change can be improved. When a scene change occurs, thepicture immediately after the scene change is handled as an intrapicture, and motion-compensation prediction is performed with the intrapicture being the first picture, whereby the number of generated intrapictures can be reduced and the amount of generated information can bereduced.

[0192] Embodiment 17

[0193]FIG. 50 is a block diagram showing the configuration of a videoinformation recording/reproducing apparatus according to the invention.In FIG. 50, the reference numerals 101 to 111 are the same as those usedin the conventional apparatus in FIG. 17 or 23.

[0194] The recording operation will be described. A video signal inputto the A/D converter 101 is converted into a digital video signal, andoutput to the high-efficiency encoder 110. The high-efficiency encoder110 performs the reduction of redundancy using auto-correlation of thevideo information, the human visual characteristics and the bias of thedata generation frequency, to compress the information (its detail willbe described later). The output of the high-efficiency encoder 110 issupplied to the error-correction encoder 102 in which error-correctingcodes for correcting transmission errors are added to it. In thisprocess, codes having a high error correction capability and a smallinformation amount to be added are used, in order to perform ahigh-density recording and because even a small error in compressedinformation exerts an influence over a wide range. The data to whicherror-correcting codes have been added are modulated by the modulator103 to a signal suitable for magnetic heads 106 and the magnetic tape105. The modulator 103 also performs other operations such assuppression of DC and low-frequency components for the azimuthrecording, and the addition of a tracking signal which assists the tracefunction of the magnetic heads 106. The record signal which has beenmodulated by the modulator 103 is recorded through the magnetic heads106 on the magnetic tape 105. The magnetic heads 106 are mounted on therotary head drum 104 so as to be rotated by the rotation of the drum104. The so-called helical scanning recording is conducted on themagnetic tape 105.

[0195] Next, the reproduction operation will be described. The signalwhich has been recorded by the helical scanning on the magnetic tape 105is picked up by the magnetic heads 106 mounted on the rotary head drum104, and then demodulated by the demodulator 107. The demodulated signalis subjected to the error detection and error correction by theerror-correction decoder 108. The error-corrected data are expanded bythe high-efficiency decoder 111 to be changed from the compressed codesto the original digital video signal. The reconstructed digital videosignal is converted to an analog video signal by the D/A converter 109,and then output.

[0196]FIG. 51 is a diagram showing an example of the tape format inEmbodiment 17. Video information of four fields({720+360×2}×480×4/2=11.06 Mbits) is compression-encoded to about 1.3Mbits and then recorded together with an audio signal and errorcorrection codes in ten tracks. In the guardbandless recording using theazimuth method, the area recording density is 2.5 μm²/bit.

[0197]FIG. 52 is a block diagram showing the internal configuration ofthe high-efficiency encoder 110 in FIG. 50. In FIG. 52, 301 designates asubtracter which outputs the difference between an input original signaland a prediction signal, 302 designates a first switch which selectseither of the input original signal and the output of the subtracter301, 303 designates a DCT circuit which performs the orthogonaltransform of DCT, 304 designates a quantizing circuit which quantizesdata to be encoded, and 305 designates a variable-length encoder whichassigns a short code to a data of a high frequency so as to eliminatethe statistical redundancy of data. The elements 306 to 311 constitute alocal decoder for obtaining a prediction signal. The reference numeral306 designates an inverse quantizing circuit which restores thequantized data, 307 designates an inverse DCT circuit which performs theinverse DCT, 308 designates an adder which adds the prediction signal tothe difference signal to reconstruct the original signal, 309 designatesa video memory which stores local-reconstructed video data, 310designates a motion-compensation circuit which detects motion from theinput original signal and outputs the next prediction data, and 311designates a second switch which switches data to be input to the adder308.

[0198] The operation of the high-efficiency encoder 110 will bedescribed. The initial field of a recording unit block is encoded as anintra-field which does not use the interplane prediction. Since thefirst switch 302 selects the upper contact, an input digital videosignal is subjected to orthogonal transform by the DCT circuit 303. Thetransformed data are quantized by the quantizing circuit 304, andencoded by the variable-length encoder 305 to a variable-length codesuch as a Huffman code, to be output. At the same time, the quantizeddata are inverse-quantized by the inverse quantizing circuit 306, andthen supplied to the inverse DCT circuit 307. In the inverse DCT circuit307, the orthogonal-transformed data are inverted to the original videodata and then output to the adder 308. In the intra-field, the secondswitch 311 also selects the upper contact so that the one input of theadder 308 is zero. Therefore, the output of the inverse DCT circuit 307is supplied as it is to the video memory 309 to be stored thereinto.

[0199] In encoding of the next field, interplane prediction is used. Ininterplane prediction, both the first and second switches 302 and 311select their lower terminal. An input digital video signal enters thesubtracter 301 and motion-compensation circuit 310. Themotion-compensation circuit 310 compares the stored picture with theinput picture, and outputs motion vectors of the input picture and aprediction picture to be used in the predictive encoding. The subtracter301 calculates the difference between the input picture and theprediction picture, and output it as a prediction difference signal tothe DCT circuit 303. Compared with a raw video signal, a predictiondifference signal has a smaller information amount as the predictionaccuracy becomes higher. For example, a prediction difference signal foran entirely still picture is zero. In the same manner as the initialfield, the data input to the DCT circuit 303 are subjected by the DCTcircuit 303 and quantizing circuit 304 to the orthogonal transform andquantization, and then converted into variable-length codes by thevariable-length encoder 305 to be output. On the other hand, thequantized data are supplied through the inverse quantizing circuit 306to the inverse DCT circuit 307 to be subjected to the inversequantization and inverse orthogonal transform, and then sent to theadder 308. Since the prediction picture used in the process of obtainingthe prediction difference is supplied to the other input terminal ofadder 308, the output of the adder 308 is the same as the input picture.The output of the adder 308 is stored into the video memory 309. In thesame manner as described above, the process of encoding n fields ispursued.

[0200]FIG. 53 shows an example of the variation of the amount ofgenerated codes for each frame. In this example, it will be noted thatan intra-field which does not use the interplane prediction is placedafter every seven fields, thereby increasing the information amount.FIG. 54 shows an example of the relationship between recordedinformation in each field and the writing in tracks. In this example,data of four fields are recorded in ten tracks. The data amount of onefield may not be an integral multiple of the recording capacity of atrack.

[0201] Embodiment 18

[0202] In Embodiment 17, data of four fields are recorded in ten tracks.It is not necessary to always record data in ten tracks, and data may berecorded in eight or six tracks depending on the amount of informationto be recorded.

[0203] As described above, in Embodiments 17 and 18, since signals of aplurality of fields or frames are collected in one recording unit to berecorded in a predetermined number of tracks, all of the recorded fieldscan be reconstructed by performing reproduction processing on apredetermined number of tracks. Therefore, the embodiments can cope withthe special reproduction and edition required in a VTR. Since the numberof tracks to be used in the recording is selected depending on theamount of information to be recorded, there is no wasted track, therebyrecording and reproduction can be performed for a long period.Furthermore, since it is not necessary to control information to berecorded so as to match the recording capacity of each track, there isno useless portion in each track, with the result being that recordingcan be performed efficiently. Moreover, since an intra-picture whichdoes not use interplane prediction surely exists in each recording unit,a reconstructed picture can be easily obtained even in a specialreproduction such as a speed search, and the amount of information to berecorded can be reduced as compared with a prediction picture which usesthe interplane prediction.

[0204] As this invention may be embodied in several forms withoutdeparting from the spirit of essential characteristics thereof, thepresent embodiment is therefore illustrative and not restrictive, sincethe scope of the invention is defined by the appended claims rather thanby the description preceding them, and all changes that fall withinmetes and bounds of the claims, or equivalence of such metes and boundsthereof are therefore intended to be embraced by the claims.

What is claimed is:
 1. A high-efficiency encoder comprising: means forsegmenting a digitized video signal into blocks of a predetermined size,and for encoding them; and prediction means for creating an intra-fieldor an intra-frame in which motion-compensation prediction is notperformed, for every n fields or frames (n is an arbitrary integer notless than 2), and for performing motion-compensation prediction with anintra-field or an intra-frame on each block for the succeeding (n−1)fields or (n−1) frames.
 2. A high-efficiency encoder comprising: meansfor segmenting a digitized video signal into blocks of a predeterminedsize, and for encoding them; prediction means for creating anintra-field or an intra-frame in which motion-compensation prediction isnot performed, for every n fields or frames (n is an arbitrary integernot less than 2), and for performing motion-compensation prediction withan intra-field or an intra-frame on each block for the succeeding (n−1)fields or (n−1) frames; means for comparing a first information amountof each block with a second information amount of each block, the firstinformation amount being obtained in a case that motion-compensationprediction is performed, the second information amount being obtained ina case that motion-compensation prediction is not performed; andselection means for, when said second information amount of the block issmaller than said first information amount of the block, selecting aforced intra mode as a coding mode of the block.
 3. A high-efficiencyencoder comprising: encoding means for segmenting a digitized videosignal into blocks of a predetermined size, and for encoding them;prediction means for creating an intra-field or an intra-frame in whichmotion-compensation prediction is not performed, for every n fields orframes (n is an arbitrary integer not less than 2), and for performingmotion-compensation prediction with an intra-field or an intra-frame oneach block for the succeeding (n−1) fields or (n−1) frames; and meansfor comparing a first information amount of each block with a secondinformation amount of each block, the first information amount beingobtained in a case that motion-compensation prediction is performed, thesecond information amount being obtained in a case thatmotion-compensation prediction is not performed, wherein, for a field orframe where the number of blocks in which the second information amountof the block is smaller than first information amount of the block isgreater, said prediction means does not perform motion-compensationprediction, and said encoding means performs encoding as a forcedintra-field or a forced intra-frame.
 4. A high-efficiency encodercomprising: encoding means for segmenting a digitized video signal intoblocks of a predetermined size, and for encoding them; prediction meansfor creating an intra-field or an intra-frame in whichmotion-compensation prediction is not performed, for every n fields orframes (n is an arbitrary integer not less than 2), and for performingmotion-compensation prediction with an intra-field or an intra-frame oneach block for the succeeding (n−1) fields or (n−1) frames; and meansfor comparing a first information amount of each block with a secondinformation amount of each block, the first information amount beingobtained in a case that motion-compensation prediction is performed, thesecond information amount being obtained in a case thatmotion-compensation prediction is not performed, wherein, for a field orframe where the number of blocks in which the second information amountof the block is smaller than the first information amount of the blockare greater, said prediction means does not perform motion-compensationprediction, and said encoding means performs encoding as a forcedintra-field or a forced intra-frame, and, for at least (n−1) fields or(n−1) frames following said forced intra mode, said prediction meansperforms again motion-compensation prediction with a forced intra-fieldor a forced intra-frame.
 5. A high-efficiency encoder comprising:encoding means for segmenting a digitized video signal into blocks of apredetermined size, and for coding them; prediction means for performingmotion-compensation prediction on each block; detection means fordetecting the occurrence of a scene change in a motion-compensationprediction processing step; and means for, when a scene change occurs,switching a field or frame in the direction along which the created codeamount is reduced, the field or frame being set as a reference pictureof motion-compensation prediction.
 6. A high-efficiency encoderaccording to claim 5, wherein said detection means detects theoccurrence of a scene change, using the number of blocks of a predictionmode or intra mode selected in a field or frame.
 7. A high-efficiencyencoder according to claim 5, wherein said detection means detects theoccurrence of a scene change, using a ratio of the number of blocks,among a predetermined number of blocks, which have been processed in aprediction mode or in an intra mode, to the predetermined number.
 8. Ahigh-efficiency encoder comprising: encoding means for segmenting adigitized video signal into blocks of a predetermined size, and forencoding them; means for switching between a prediction mode in whichmotion-compensation prediction is performed on each block and an intramode in which motion-compensation prediction is not performed; and meansfor, when a scene change occurs in a motion-compensation predictionprocessing step, detecting the scene change according to a signal whichis generated from the prediction mode and the intra mode, and forswitching a field or frame in the direction along which the created codeamount is reduced, the field or frame being set as a reference pictureof motion-compensation prediction.
 9. A high-efficiency encoderaccording to claim 8, wherein the occurrence of a scene change isdetected, using the number of blocks of a prediction mode or intra modeselected in a field or frame.
 10. A high-efficiency encoder according toclaim 8, wherein the occurrence of a scene change is detected, using aratio of the number of blocks, among a predetermined number of blocks,which have been processed in a prediction mode or in an intra mode, tothe predetermined number.
 11. A high-efficiency encoder comprising:encoding means for segmenting a digitized video signal into blocks of apredetermined size, and for encoding them; prediction means forperforming motion-compensation prediction on each block; detection meansfor detecting the occurrence of a scene change in a motion-compensationprediction processing step; and means for switching a reference picturein such a manner that the field or frame immediately after the detectedscene change is surely set as a reference picture of motion-compensationprediction.
 12. A high-efficiency encoder according to claim 11, whereinsaid detection means detects the occurrence of a scene change, using thenumber of blocks of a prediction mode or intra mode selected in a fieldor frame.
 13. A high-efficiency encoder according to claim 11, whereinsaid detection means detects the occurrence of a scene change, using aratio of the number of blocks, among a predetermined number of blocks,which have been processed in a prediction mode or in an intra mode, tothe predetermined number.
 14. A high-efficiency encoder comprising:means for segmenting a digitized video signal into blocks of apredetermined size, and for encoding them; prediction means forperforming motion-compensation prediction on each block; detection meansfor detecting the occurrence of a scene change in a motion-compensationprediction processing step; and means for switching a reference picturein such a manner that a field or frame before the detected scene changeis not surely used as a reference picture of motion-compensationprediction.
 15. A high-efficiency encoder comprising: means forsegmenting a digitized video signal into blocks of a predetermined size,and for encoding them; means for setting n fields or frames (n is anarbitrary integer not less than 2) as one process unit, and forperforming a motion-compensation prediction process in such a mannerthat an intra-field or intra-frame of one field is in the process unit;and detection means for detecting the occurrence of a scene change inthe motion-compensation prediction process unit, wherein a field orframe in which a scene change is detected is set as an intra-field orintra-frame, and a plurality of intra-fields or intra-frames are in themotion-compensation prediction process unit.
 16. A high-efficiencyencoder comprising: means for segmenting a digitized video signal intoblocks of a predetermined size, and for encoding them; means for settingn fields or frames (n is an arbitrary integer not less than 2) as oneprocess unit, and for performing a motion-compensation predictionprocess in such a manner that an intra-field or intra-frame of one fieldis in the process unit; and detection means for detecting the occurrenceof a scene change in the motion-compensation prediction process unit,wherein a field or frame in which a scene change is detected is set asan intra-field or intra-frame, and as the first field or frame of a newmotion-compensation prediction process unit.
 17. A high-efficiencyencoder comprising: means for segmenting a digitized video signal intoblocks of a predetermined size, and for coding them; means for setting nfields or frames (n is an arbitrary integer not less than 2) as oneprocess unit, and for performing a motion-compensation predictionprocess in such a manner that an intra-field or intra-frame of one fieldis in the process unit; and detection means for detecting the occurrenceof a scene change in the motion-compensation prediction process unit,wherein a series of fields or frames from a field or frame in which ascene change is detected to the last field or frame of themotion-compensation prediction process unit are combined with the nextmotion-compensation prediction process unit, the motion-compensationprediction process unit is longer than n fields or frames of a usualmotion-compensation prediction process unit, and motion-compensationprediction is performed while an intra-field or intra-frame belonging tothe next motion-compensation prediction process unit is set as areference picture.
 18. A high-efficiency encoder comprising: means forsegmenting a digitized video signal into blocks of a predetermined size,and for coding them; means for setting n fields or frames (n is anarbitrary integer not less than 2) as one process unit, and forperforming a motion-compensation prediction process in such a mannerthat an intra-field or intra-frame of one field is in the process unit;and detection means for detecting the occurrence of a scene change inthe motion-compensation prediction process unit, whereinmotion-compensation prediction is performed while a field or frame inwhich a scene change is detected is set as an intra-field orintra-frame, and n fields or frames from the field or frame in which ascene change is detected are set for the next motion-compensationprediction process unit.
 19. A video information recording/reproducingapparatus for recording video information with helical scanningrecording, wherein signals of a plurality of fields or frames arecollected in one recording unit and recorded in a predetermined numberof tracks.
 20. A video information recording/reproducing apparatusaccording to claim 19, wherein an information amount of one field orframe in the case of collecting signals in one recording unit is notnecessarily an integral multiple of an information amount which can berecorded in one track.
 21. A video information recording/reproducingapparatus according to claim 20, wherein, in the signals of a pluralityof fields or frames to be recorded, an intra-field or intra-frame inwhich interplane prediction is not used and a prediction-field orprediction frame in which interplane prediction is used are mixedlypresent.
 22. A video information recording/reproducing apparatus forrecording video information with helical scanning recording, whereinsignals of a plurality of fields or frames are collected in onerecording unit, and recorded in one or more tracks which are selectedfrom a predetermined number of tracks in accordance with an informationamount to be recorded.
 23. A video information recording/reproducingapparatus according to claim 22, wherein an information amount of onefield or frame in the case of collecting signals in one recording unitis not necessarily an integral multiple of an information amount whichcan be recorded in one track.
 24. A video informationrecording/reproducing apparatus according to claim 23, wherein, in thesignals of a plurality of fields or frames to be recorded, anintra-field or intra-frame in which interplane prediction is not usedand a prediction-field or prediction frame in which interplaneprediction is used are mixedly present.